Ariel FridmanpersName, Rachel GershonpersName, Ayelet GneezypersName (2021date). “COVID-19 and vaccine hesitancy: A longitudinal study.” PLOS ONEtitle·j.
Object of study —
Research question — change in United States vaccination attitudes during the first six months of COVID-19
Contribution — evidence connecting partisan attitude divergence with risk perceptionterm·dom and information exposure
phenomenon · claim-type · cross-domain-transfer · mechanism · claim-relation · replication-status · limitation — grounded in the full text| phenomenon | changes in attitudes toward a prospective COVID-19 vaccine and toward vaccines in general during the COVID-19 health crisis — “The present research reports findings from a longitudinal study conducted during the COVID-19 health crisis, in which we measured changes in attitudes toward a prospective vaccine, as well as shifts in vaccine attitudes in general.” | |
| claim-type | existence — “We provide suggestive evidence that differential exposure to media channels and social networks could explain the observed asymmetric polarization between self-identified Democrats and Republicans.” | |
| method — evidence | — “We report results from a longitudinal survey of United States residents during six months (March 16 –August 16, 2020) of the COVID-19 pandemic.” | |
| research question — evidence | — “How do attitudes toward vaccination change over the course of a public health crisis?” | |
| contribution — evidence | — “We provide suggestive evidence that differential exposure to media channels and social networks could explain the observed asymmetric polarization between self-identified Democrats and Republicans.” | |
| mechanism | Ironically, the objection to vaccines is commonly a consequence of their effectiveness—because individuals have lower exposure to VPDs, they are less concerned about contracting them [ 9 ], which consequently leads to greater vaccine hesitancy [ 10 ]. — “Ironically, the objection to vaccines is commonly a consequence of their effectiveness—because individuals have lower exposure to VPDs, they are less concerned about contracting them [ 9 ], which consequently leads to greater vaccine hesitancy [ 10 ].” | mechanisms[0] |
| mechanism | When individuals perceive heightened risk of a threat, they become more favorable toward interventions that mitigate that threat, including vaccination (for a meta-analysis on the effect of perceived risk on intentions and behaviors, see [ 14 ]). — “When individuals perceive heightened risk of a threat, they become more favorable toward interventions that mitigate that threat, including vaccination (for a meta-analysis on the effect of perceived risk on intentions and behaviors, see [ 14 ]).” | mechanisms[1] |
| mechanism | For example, research shows that media coverage plays a significant role in determining the extent to which we take threats seriously [ 13 ]. — “For example, research shows that media coverage plays a significant role in determining the extent to which we take threats seriously [ 13 ].” | mechanisms[2] |
| mechanism | While our data does not render causal claims, it is possible that the divergence in COVID-19 threat perceptions over time among Republicans and Democrats contributes to the divergence in vaccine attitudes between these groups over time. — “While our data does not render causal claims, it is possible that the divergence in COVID-19 threat perceptions over time among Republicans and Democrats contributes to the divergence in vaccine attitudes between these groups over time.” | mechanisms[3] |
| mechanism | This trend is consistent with previous research showing that vaccine hesitancy is related to perceived risk of a threat; when a VPD threat level is low, individuals are less motivated to take preventative action (i.e., immunize; for a review, see [ 42 ]). — “This trend is consistent with previous research showing that vaccine hesitancy is related to perceived risk of a threat; when a VPD threat level is low, individuals are less motivated to take preventative action (i.e., immunize; for a review, see [ 42 ]).” | mechanisms[4] |
| mechanism | Our data offers one potential explanation for the polarization of threat perception: Republican and Democratic participants in our study reported consuming different sources of information. — “Our data offers one potential explanation for the polarization of threat perception: Republican and Democratic participants in our study reported consuming different sources of information.” | mechanisms[5] |
| mechanism | Extant work describes these platforms as echo chambers [ 44 , 45 ], which may exacerbate partisan exposure to news and information. — “Extant work describes these platforms as echo chambers [ 44 , 45 ], which may exacerbate partisan exposure to news and information.” | mechanisms[6] |
| mechanism | According to Dr. Heidi Larson, an expert on vaccine hesitancy and founder of the Vaccine Confidence Project, misinformation regarding vaccinations is more likely to take root when individuals do not trust the information source [ 46 ]. — “According to Dr. Heidi Larson, an expert on vaccine hesitancy and founder of the Vaccine Confidence Project, misinformation regarding vaccinations is more likely to take root when individuals do not trust the information source [ 46 ].” | mechanisms[7] |
| supports | the review of vaccine hesitancy and perceived threat [42] — “This trend is consistent with previous research showing that vaccine hesitancy is related to perceived risk of a threat; when a VPD threat level is low, individuals are less motivated to take preventative action (i.e., immunize; for a review, see [ 42 ]).” | claim_relations[0] |
| contradicts | threat-induced social-cohesion and cooperation studies [36–38] — “Contrary to the polarization observed in our data, social and behavioral scientists have long argued that groups facing threats often come together, demonstrating stronger social cohesion [ 36 ], and more cooperative behaviors [ 37 , 38 ].” | claim_relations[1] |
| sense | vaccine-related outcomes comprise COVID-19 vaccine attitudes, general vaccine attitudes, and flu-shot intentions — “We report results for three different vaccination-related measures: attitudes toward a COVID-19 vaccine, general vaccination attitudes, and flu shot intentions.” | senses[0] |
| control | Data and analysis plan Only participants with non-missing and non-duplicated responses were included in the analyses (see S1 Appendix for additional details). — rules out missing and duplicate survey records — “Data and analysis plan Only participants with non-missing and non-duplicated responses were included in the analyses (see S1 Appendix for additional details).” | controls[0] |
| control | All regression results include individual-level fixed effects, and standard errors are clustered at the individual level, to adjust for within-person correlation. — rules out within-person error correlation — “All regression results include individual-level fixed effects, and standard errors are clustered at the individual level, to adjust for within-person correlation.” | controls[1] |
| control | We used this approach to control for the impact of omitted or unobserved time-invariant variables. — rules out time-invariant individual confounding — “We used this approach to control for the impact of omitted or unobserved time-invariant variables.” | controls[2] |
| control | For these regressions, and moving forward, all results included interactions between wave and political party as well as interactions for wave and age, and wave and SES, to control for potentially different time trends associated with these variables. — rules out different temporal trends by party, age, and socioeconomic status — “For these regressions, and moving forward, all results included interactions between wave and political party as well as interactions for wave and age, and wave and SES, to control for potentially different time trends associated with these variables.” | controls[3] |
| control | Attrition To rule out differential attrition, we tested whether the composition of our sample (i.e., age, gender, and political party) changed over time (see S1 Table ). — rules out changes in panel composition across waves — “Attrition To rule out differential attrition, we tested whether the composition of our sample (i.e., age, gender, and political party) changed over time (see S1 Table ).” | controls[4] |
| control | Specifically, we tested whether participants who responded to waves 2–6 were significantly different at baseline (wave 1) from the full sample at baseline. — rules out baseline differences among retained respondents — “Specifically, we tested whether participants who responded to waves 2–6 were significantly different at baseline (wave 1) from the full sample at baseline.” | controls[5] |
| control | Trust in broad institutions The measures of trust in media, local government, and federal government were not highly correlated ( α = .66), and were therefore analyzed separately. — rules out invalid aggregation of distinct institutional-trust measures — “Trust in broad institutions The measures of trust in media, local government, and federal government were not highly correlated ( α = .66), and were therefore analyzed separately.” | controls[6] |
| no type for this | risk perception → vaccine hesitancy — — “This trend is consistent with previous research showing that vaccine hesitancy is related to perceived risk of a threat; when a VPD threat level is low, individuals are less motivated to take preventative action (i.e., immunize; for a review, see [ 42 ]).” | unhoused_relations[0] |
| limitation | Census (2019) [ 31 ] estimates, our sample over-represents White and Asian individuals, and under-represents Black or African American individuals and other racial groups. — “Census (2019) [ 31 ] estimates, our sample over-represents White and Asian individuals, and under-represents Black or African American individuals and other racial groups.” | limitations[0] |
| limitation | While our data does not render causal claims, it is possible that the divergence in COVID-19 threat perceptions over time among Republicans and Democrats contributes to the divergence in vaccine attitudes between these groups over time. — “While our data does not render causal claims, it is possible that the divergence in COVID-19 threat perceptions over time among Republicans and Democrats contributes to the divergence in vaccine attitudes between these groups over time.” | limitations[1] |
| limitation | While trust in media or media exposure may be driving COVID-19 threat perceptions and vaccine attitudes, there are many other possible explanatory factors that are not captured by our data or analyses. — “While trust in media or media exposure may be driving COVID-19 threat perceptions and vaccine attitudes, there are many other possible explanatory factors that are not captured by our data or analyses.” | limitations[2] |
| limitation | Further, although we included individual-level fixed-effects which control for all time invariant participant characteristics, and controlled for different trends by age and SES, we cannot rule out the possibility that other factors (e.g., educational attainment or population density) may have influenced the observed trends. — “Further, although we included individual-level fixed-effects which control for all time invariant participant characteristics, and controlled for different trends by age and SES, we cannot rule out the possibility that other factors (e.g., educational attainment or population density) may have influenced the observed trends.” | limitations[3] |
| limitation | Finally, as our data collection began after the onset of COVID-19, it is possible that the trend we observe for Republicans represents a return to a pre-pandemic baseline of vaccine-related attitudes. — “Finally, as our data collection began after the onset of COVID-19, it is possible that the trend we observe for Republicans represents a return to a pre-pandemic baseline of vaccine-related attitudes.” | limitations[4] |
finding · quantity · variable · definition · key-term — grounded in the full text| finding | Contrary to past research suggesting that the increased salience of a disease threat should improve attitudes toward vaccines, we observed a decrease in intentions of getting a COVID-19 vaccine when one becomes available. — “Contrary to past research suggesting that the increased salience of a disease threat should improve attitudes toward vaccines, we observed a decrease in intentions of getting a COVID-19 vaccine when one becomes available.” | content_tags.findings[0] |
| finding | We further found a decline in general vaccine attitudes and intentions of getting the influenza vaccine. — “We further found a decline in general vaccine attitudes and intentions of getting the influenza vaccine.” | content_tags.findings[1] |
| finding | Specifically, participants who identify as Republican showed a decrease in their intention to get the COVID-19 vaccine and the influenza vaccine as well as a general decrease in pro-vaccine attitudes, whereas Democrats’ responses to these measures did not show a significant change during this period. — “Specifically, participants who identify as Republican showed a decrease in their intention to get the COVID-19 vaccine and the influenza vaccine as well as a general decrease in pro-vaccine attitudes, whereas Democrats’ responses to these measures did not show a significant change during this period.” | content_tags.findings[2] |
| finding | There was no significant change in mean political identity from wave 1 to waves 2–6 (see S2 Table ). — “There was no significant change in mean political identity from wave 1 to waves 2–6 (see S2 Table ).” | content_tags.findings[3] |
| finding | Among all demographic characteristics, separating time trends by political affiliation (by adding an interaction term) attained the greatest adjusted within-R 2 in explaining vaccination attitude measures. — “Among all demographic characteristics, separating time trends by political affiliation (by adding an interaction term) attained the greatest adjusted within-R 2 in explaining vaccination attitude measures.” | content_tags.findings[4] |
| finding | All measures showed a decreasing trend (Ps < .001, except flu shot intentions where p = .05) for the 6-month duration of the study, indicating a reduction in pro-vaccination attitudes and intention to get vaccinated (COVID-19 and influenza vaccines). — “All measures showed a decreasing trend (Ps < .001, except flu shot intentions where p = .05) for the 6-month duration of the study, indicating a reduction in pro-vaccination attitudes and intention to get vaccinated (COVID-19 and influenza vaccines).” | content_tags.findings[5] |
| finding | We found no other systematic pattern of attrition among our participants. — “We found no other systematic pattern of attrition among our participants.” | content_tags.findings[6] |
| finding | Our analyses identify political ideology as the best predictor of the decreasing time trend across our three vaccine-related attitudes and intentions measures. — “Our analyses identify political ideology as the best predictor of the decreasing time trend across our three vaccine-related attitudes and intentions measures.” | content_tags.findings[7] |
| finding | The different trends we observe for Democrats and Republicans with respect to trust in the media may explain the divergence in perceived threat and vaccine attitudes between these groups over time (see General discussion ). — “The different trends we observe for Democrats and Republicans with respect to trust in the media may explain the divergence in perceived threat and vaccine attitudes between these groups over time (see General discussion ).” | content_tags.findings[8] |
| finding | Our data offers one potential explanation for the polarization of threat perception: Republican and Democratic participants in our study reported consuming different sources of information. — “Our data offers one potential explanation for the polarization of threat perception: Republican and Democratic participants in our study reported consuming different sources of information.” | content_tags.findings[9] |
| finding | For example, our analyses reveal that polarization on vaccine measures—both attitudes and intentions—is driven primarily by self-identified Republicans’ gradual movement away from their initial responses whereas Democrats’ responses remained largely stable. — “For example, our analyses reveal that polarization on vaccine measures—both attitudes and intentions—is driven primarily by self-identified Republicans’ gradual movement away from their initial responses whereas Democrats’ responses remained largely stable.” | content_tags.findings[10] |
| quantity | We report results from a longitudinal survey of United States residents during six months (March 16 –August 16, 2020) of the COVID-19 pandemic. — “We report results from a longitudinal survey of United States residents during six months (March 16 –August 16, 2020) of the COVID-19 pandemic.” | content_tags.quantities[0] |
| quantity | Participants were paid 30 cents for wave 1, 40 cents for wave 2, and 60 cents for waves 3 and 4, $1.00 for wave 5, and $1.20 for wave 6. — “Participants were paid 30 cents for wave 1, 40 cents for wave 2, and 60 cents for waves 3 and 4, $1.00 for wave 5, and $1.20 for wave 6.” | content_tags.quantities[1] |
| quantity | The sample size for the first wave was 1,018, and the number of participants ranged from 608–762 on subsequent waves (see S1 Table for attrition details). — “The sample size for the first wave was 1,018, and the number of participants ranged from 608–762 on subsequent waves (see S1 Table for attrition details).” | content_tags.quantities[2] |
| quantity | The first wave sample included participants from all 50 states (except Wyoming) and Washington D.C., with an age range of 18 to 82 years old (mean = 38.48, median = 35). — “The first wave sample included participants from all 50 states (except Wyoming) and Washington D.C., with an age range of 18 to 82 years old (mean = 38.48, median = 35).” | content_tags.quantities[3] |
| quantity | Approximately half (53%) identified as male, 46% as female, and.6% as other. — “Approximately half (53%) identified as male, 46% as female, and.6% as other.” | content_tags.quantities[4] |
| quantity | The racial makeup in our sample was: 80% White, 9% Asian, 6% Black or African American, 4% multiple racial or ethnic identities, and 1% other. — “The racial makeup in our sample was: 80% White, 9% Asian, 6% Black or African American, 4% multiple racial or ethnic identities, and 1% other.” | content_tags.quantities[5] |
| quantity | In wave 1, 62% identified as Democrats and 38% identified as Republican, which is consistent with results from the most recent General Social Survey (GSS) [ 32 ]. — “In wave 1, 62% identified as Democrats and 38% identified as Republican, which is consistent with results from the most recent General Social Survey (GSS) [ 32 ].” | content_tags.quantities[6] |
| quantity | The most commonly checked news source for Republicans was Fox News (Republicans: 50%, Democrats: 8%; χ 2 = 164.55, p < .001) and for Democrats was CNN (Democrats: 47%, Republicans: 23%, χ 2 = 43.08, p < .001, see S6 Table ). — “The most commonly checked news source for Republicans was Fox News (Republicans: 50%, Democrats: 8%; χ 2 = 164.55, p < .001) and for Democrats was CNN (Democrats: 47%, Republicans: 23%, χ 2 = 43.08, p < .001, see S6 Table ).” | content_tags.quantities[7] |
| definition | COVID-19 vaccination intention — “Questions and measures Our primary measure of interest was participants’ stated intention to get the COVID-19 vaccine when it becomes available.” | content_tags.definitions[0] |
| definition | party affiliation — “We classified participants as Democrats or Republicans using wave 1 political party affiliation.” | content_tags.definitions[2] |
| definition | perceived COVID-19 threat — “A three-item construct ( α = .82) was created, with greater perceived threat about COVID-19.” | content_tags.definitions[3] |
| definition | general vaccination attitudes — “We report results for three different vaccination-related measures: attitudes toward a COVID-19 vaccine, general vaccination attitudes, and flu shot intentions.” | content_tags.definitions[5] |
| definition | flu shot intentions — “We report results for three different vaccination-related measures: attitudes toward a COVID-19 vaccine, general vaccination attitudes, and flu shot intentions.” | content_tags.definitions[6] |
| concept | vaccine hesitancy — “Vaccine hesitancy has led to a decline in vaccine uptake and to an increase in the prevalence of vaccine-preventable diseases (VPDs) [ 7 , 8 ].” | concepts[0] |
| concept | risk perception — “When individuals perceive heightened risk of a threat, they become more favorable toward interventions that mitigate that threat, including vaccination (for a meta-analysis on the effect of perceived risk on intentions and behaviors, see [ 14 ]).” | concepts[1] |
| overall vaccine-measure time trends | — “All measures showed a decreasing trend (Ps < .001, except flu shot intentions where p = .05) for the 6-month duration of the study, indicating a reduction in pro-vaccination attitudes and intention to get vaccinated (COVID-19 and influenza vaccines).” | content_tags.statistics[0] |
| party-specific time trends | — “An analysis of responses by political affiliation revealed that the observed decreasing trend in all three vaccine measures was mostly driven by participants who identified as Republican (all Ps < .05), whereas Democrats’ responses showed either no significant trend (for COVID-19 vaccination and flu shot intentions: Ps >.67) or a significantly less negative time trend (general vaccination: p < .001).” | content_tags.statistics[1] |
| party-specific vaccination time trend | — “Among Democrats, there was no significant time trend ( β = .02, SE = .04, p >.67) whereas Republicans’ responses followed a decreasing time trend ( β = -.09, SE = .05, p = .046).” | content_tags.statistics[2] |
| Republican flu-shot trend | — “Republicans’ responses, however, revealed a decreasing time trend ( β = -.12, SE = .04, p = .005), and the two trends were significantly different from each other ( β = -.12, SE = .02, p < .001).” | content_tags.statistics[3] |
| party-strength interaction | — “Our analyses revealed an interaction with political affiliation strength among Republicans, whereby participants who identified as more strongly Republican had a more negative time trend ( β = -.05, SE = .02, p = .027).” | content_tags.statistics[4] |
| institutional-trust reliability | — “Trust in broad institutions The measures of trust in media, local government, and federal government were not highly correlated ( α = .66), and were therefore analyzed separately.” | content_tags.statistics[5] |
| perceived-threat reliability | — “A three-item construct ( α = .82) was created, with greater perceived threat about COVID-19.” | content_tags.statistics[6] |
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Abstract. Abstract How do attitudes toward vaccination change over the course of a public health crisis? We report results from a longitudinal surveyterm·mth of United States residents during six months (March 16 –August 16, 2020) of the COVID-19 pandemic. Contrary to past research suggesting that the increased salience of a disease threat should improve attitudes toward vaccines, we observed a decrease in intentions of getting a COVID-19 vaccine when one becomes available. We further found a decline in general vaccine attitudes and intentions of getting the influenza vaccine. Analyses of heterogeneity indicated that this decline is driven by participants who identify as Republicans, who showed a negative trend in vaccine attitudes and intentions, whereas Democrats remained largely stable. Consistent with research on risk perceptionterm·dom and behavior, those with less favorable attitudes toward a COVID-19 vaccination also perceived the virus to be less threatening. We provide suggestive evidence that differential exposure to media channels and social networks could explain the observed asymmetric polarization between self-identified Democrats and Republicans. Introduction Vaccinations are among the most important public health tools for reducing the spread and harm caused by dangerous diseases [ 1 ]. The World Health Organization estimates that vaccines prevented at least 10 million deaths between 2010–2015 worldwide [ 2 ]. Despite considerable evidence showing vaccines are safe [ 3 , 4 ], there is increasing skepticism toward vaccination [ 5 , 6 ]. Vaccine hesitancyterm·dom has led to a decline in vaccine uptake and to an increase in the prevalence of vaccine-preventable diseases (VPDs) [ 7 , 8 ]. Ironically, the objection to vaccines is commonly a consequence of their effectiveness—because individuals have lower exposure to VPDs, they are less concerned about contracting them [ 9 ], which consequently leads to greater vaccine hesitancyterm·dom [ 10 ]. The COVID-19 pandemic has created a new reality where individuals are faced with a previously unknown disease and its effects, providing a unique opportunity to investigate vaccine attitudes during a period of heightened disease salience. The present research reports findings from a longitudinal study conducted during the COVID-19 health crisis, in which we measured changes in attitudes toward a prospective vaccine, as well as shifts in vaccine attitudes in general. Factors influencing vaccine attitudes and behaviors Past research has identified a variety of situational and individual-level factors that influence vaccine attitudes and behavior, the most prominent of which are risk perceptionsterm·dom and demographic characteristics. Assessments of risk are influenced by both cognitive evaluations (i.e., objective features of the situation such as probabilities of outcomes) and affective reactions [ 11 ], as well as by contextual factors (e.g., the information that is most available or salient at the time [ 12 ]). For example, research shows that media coverage plays a significant role in determining the extent to which we take threats seriously [ 13 ]. When individuals perceive heightened risk of a threat, they become more favorable toward interventions that mitigate that threat, including vaccination (for a meta-analysis on the effect of perceived risk on intentions and behaviors, see [ 14 ]). In the case of COVID-19, this would suggest more positive attitudes toward a vaccine and greater likelihood to get vaccinated. Indeed, research suggests that individuals should exhibit a greater interest in vaccinations during a pandemic because disease threat is more salient [ 15 ]. Past efforts to improve vaccine attitudes have had limited success or even backfired; for example, messages refuting claims about the link between vaccines and autism, as well as messages featuring images of children who were sick with VPDs, had negative effects on vaccine attitudes among those who were already hesitant to vaccinate [ 16 ]. In contrast, messaging that increases disease threat salience has shown promise in reducing vaccine hesitancyterm·dom [ 5 ], and there is evidence suggesting that increased threat salience for a particular disease may also increase intentions to vaccinate for other diseases [ 17 ]. Building on these findings, we expected to find an increase in pro-vaccine attitudes and in individuals’ interest in a COVID-19 vaccine when the perceived threat of the COVID-19 virus increased. Vaccine attitudes are also influenced by a variety of demographic and ideological factors (for a review, see [ 18 ]). For example, perceptions of vaccine risk differ among individuals of different ethnic backgrounds [ 19 ], and there is extant work demonstrating a positive correlation between socioeconomic status (SES) and vaccine hesitancyterm·dom [ 20 , 21 ]. Socio-demographic factors are also linked to vaccine-related behaviors: among college students, those whose parents have attained a higher level of education are more likely to get immunized [ 22 ], and researchers have identified age as a predictor for receiving the influenza vaccine [ 23 ]. Political ideology is another well-documented determinant of vaccine-related attitudes and behaviors. Despite a common belief that liberals tend toward anti-vaccination attitudes in the United States, there is strong evidence that this trend is more present among conservatives [ 24 , 25 ]. According to a recent Gallup Poll, Republicans are twice as likely to believe the widely debunked myth that vaccines cause autism [ 26 ]. Recent work has shown that exposure to anti-vaccination tweets by President Trump—the first known U.S. president to publicly express anti-vaccination attitudes—has led to increased concern about vaccines among his supporters [ 27 ]. Based on these findings, and in conjunction with the sentiments expressed by the White House that diminished the significance of the pandemic [ 28 ], we expected to find diverging trends between Democrats and Republicans. The current research We collected vaccine-related attitudes of individuals living in the U.S. over a six-month period. Beginning in March 2020, we elicited attitudes from a cohort of the same individuals every month. We began data collection before any COVID-19 lockdown measures were in place (i.e., prior to the nation’s first shelter-in-place order [ 29 ]). Hence, our data spans the early phase of the pandemic, when there were fewer than 2,000 total confirmed cases in the U.S., through the following six months, at which point cumulative cases reached over 5.3 million [ 30 ]. Despite our prediction—that a public health crisis would increase disease threat, consequently increasing pro-vaccine attitudes and interest in vaccination—our data show an overall decrease in favorable attitudes toward vaccines. A closer look at the data revealed that political orientation explains more variance than any other socio-demographic variable. Specifically, participants who identify as Republican showed a decrease in their intention to get the COVID-19 vaccine and the influenza vaccine as well as a general decrease in pro-vaccine attitudes, whereas Democrats’ responses to these measures did not show a significant change during this period. Our work is the first, to our knowledge, to longitudinally measure individuals’ attitudes toward vaccines. In doing so, our findings advance the understanding of how vaccine attitudes might change during an unprecedented public health crisis, revealing a strong association between political party affiliationterm·def and vaccine attitudes. Methods Participants We recruited a panel of U.S. residents on Amazon’s Mechanical Turk platform to respond to multiple surveyterm·mth waves. To incentivize completion of all waves, we informed participants their payment would increase for subsequent surveysterm·mth. Participants were paid 30 cents for wave 1, 40 cents for wave 2, and 60 cents for waves 3 and 4, $1.00 for wave 5, and $1.20 for wave 6. In addition, participants were informed that those who completed the first three waves would enter a $100 raffle. The median surveyterm·mth completion time was 5.5 minutes. The sample size for the first wave was 1,018, and the number of participants ranged from 608–762 on subsequent waves (see S1 Table for attrition details). This project was certified as exempt from IRB review by the University of California, San Diego Human Research Protections Program (Project #191273XX). Our panel represents the broad and diverse population of the United States. The first wave sample included participants from all 50 states (except Wyoming) and Washington D.C., with an age range of 18 to 82 years old (mean = 38.48, median = 35). Approximately half (53%) identified as male, 46% as female, and.6% as other. The racial makeup in our sample was: 80% White, 9% Asian, 6% Black or African American, 4% multiple racial or ethnic identities, and 1% other. Relative to the U.S. Census (2019) [ 31 ] estimates, our sample over-represents White and Asian individuals, and under-represents Black or African American individuals and other racial groups. We elicited political affiliation using a 6-point Likert scale, ranging from Strongly Republican to Strongly Democratic. In wave 1, 62% identified as Democrats and 38% identified as Republican, which is consistent with results from the most recent General Social Surveyterm·mth (GSS) [ 32 ]. There was no significant change in mean political identity from wave 1 to waves 2–6 (see S2 Table ). We classified participants as Democrats or Republicans using wave 1 political party affiliationterm·def. See S2 Appendix for additional details about the correlation of political party affiliationterm·def with age, gender, and SES. Questions and measures Our primary measure of interest was participants’ stated intention to get the COVID-19 vaccine when it becomes available. We were also interested in their perceptions of COVID-19 threat, general vaccination attitudesterm·def, and intention to get the flu shot. For all measures, except flu shot intentionsterm·def, we combined multiple items to create a composite measure (see S2 Table for specific questions and construct compositions). Questions designed to measure general vaccination attitudesterm·def were adapted from prior work [ 33 ]. Additional measures of interest were participants’ trust in broad institutions (media, local government, and federal government). These trust measures followed different trends from each other, and therefore were not combined. At the end of the surveyterm·mth, participants responded to demographic questions. We retained all questions used in wave 1 throughout all six waves (our surveyterm·mth included additional items not reported in this paper; see S2 and S3 Tables for a complete list of measured items). Data and analysis plan Only participants with non-missing and non-duplicated responses were included in the analyses (see S1 Appendix for additional details). For all outcomes of interest, we tested for linear trends over time using a fixed effects regression specification [ 34 ]. All regression results include individual-level fixed effects, and standard errors are clustered at the individual level, to adjust for within-person correlation. We used this approach to control for the impact of omitted or unobserved time-invariant variables. P-values are not adjusted for multiple testing (see [ 35 ]). All analyses were conducted using R (version 4.0.2), and regressions were run using the package “fixest” (version 0.6.0). All materials, data, and additional analyses including robustness checks can be found here: https://osf.io/kgvdy/ . Results We report results for three different vaccination-related measures: attitudes toward a COVID-19 vaccine, general vaccination attitudesterm·def, and flu shot intentionsterm·def. All measures showed a decreasing trend (Ps < .001, except flu shot intentionsterm·def where p = .05) for the 6-month duration of the study, indicating a reduction in pro-vaccination attitudes and intention to get vaccinated (COVID-19 and influenza vaccines). See S4 Table for full results of all regressions. Heterogeneity in trend by political party To better understand whether the decline in vaccine attitudes over time was driven by a particular factor, we used a data-driven approach, regressing all demographic characteristics on vaccine attitudes, in separate regressions. These demographics included education, income, SES, race, gender, an item measuring whether participants considered themselves to be a minority, whether the participant has children, and political party. Education, income, and SES were median split; race and gender were dummy coded; and political party affiliationterm·def was dichotomized into Democrat or Republican. Among all demographic characteristics, separating time trends by political affiliation (by adding an interaction term) attained the greatest adjusted within-R 2 in explaining vaccination attitude measures. In other words, political party affiliationterm·def explains the greatest within-individual variation in vaccine attitudes over time. An analysis of responses by political affiliation revealed that the observed decreasing trend in all three vaccine measures was mostly driven by participants who identified as Republican (all Ps < .05), whereas Democrats’ responses showed either no significant trend (for COVID-19 vaccination and flu shot intentionsterm·def: Ps >.67) or a significantly less negative time trend (general vaccination: p < .001). For these regressions, and moving forward, all results included interactions between wave and political party as well as interactions for wave and age, and wave and SES, to control for potentially different time trends associated with these variables. In each regression we also tested whether the strength of political affiliation moderates the observed results, and we reported the result when it did. We also conducted ANOVAs to compare mean responses for the outcomes of interest between Democrats and Republicans, separately for each wave (see S5 Table ). COVID-19 vaccination attitudes ( Fig 1 , Panel A) Fig 1 Vaccination attitudes and intentions by political affiliation (March–August 2020). Points represent means, and error bars represent 95% confidence intervals. All scale responses range from 1 to 7. A two-item construct ( r = .78) was created, with greater values corresponding to more favorable responses. In wave 1, Democrats ( M = 5.39, SD = 1.55) had more favorable attitudes toward a COVID-19 vaccine than Republicans ( M = 4.57, SD = 1.76; t = -7.38, p < .001, d = -.48, 95% CI = [-.61, -.35]). Among Democrats, there was no significant time trend ( β = .02, SE = .04, p >.67) whereas Republicans’ responses followed a decreasing time trend ( β = -.09, SE = .05, p = .046). These trends were significantly different from each other ( β = -.11, SE = .02, p < .001). General vaccination attitudesterm·def ( Fig 1 , Panel B) A ten-item construct ( α = .95) was created, with greater values corresponding to a more positive attitude toward vaccination in general. In wave 1, Democrats ( M = 5.83, SD = 1.15) expressed more favorable general vaccination attitudesterm·def than Republicans ( M = 5.17, SD = 1.31; t = -7.91, p < .001, d = -.52, 95% CI = [-.66, -.39]). Although both Democrats and Republicans had a decreasing time trend (Democrats: β = -.04, SE = .02, p = .029; Republicans: β = -.09, SE = .02, p < .001), the trend for Republicans was significantly more negative ( β = -.04, SE = .01, p < .001). Flu shot intentionsterm·def ( Fig 1 , Panel C) We asked participants whether they plan to get the flu shot next year, with greater values indicating greater intentions. In wave 1, Democrats ( M = 4.84, SD = 2.34) indicated greater intentions to vaccinate against the flu than Republicans ( M = 4.35, SD = 2.39; t = -3.15, p = .002, d = -.21, 95% CI = [-.34, -.08]). Among Democrats, there was no significant time trend ( β = .01, SE = .04, p = .86), suggesting their vaccination intentions remained largely stable. Republicans’ responses, however, revealed a decreasing time trend ( β = -.12, SE = .04, p = .005), and the two trends were significantly different from each other ( β = -.12, SE = .02, p < .001). Our analyses revealed an interaction with political affiliation strength among Republicans, whereby participants who identified as more strongly Republican had a more negative time trend ( β = -.05, SE = .02, p = .027). This interaction was not significant for Democrats ( β = -.02, SE = .01, p = .19). Perceived threat of COVID-19 ( Fig 2 ) Fig 2 Perceived threat of COVID-19 by political affiliation (March–August 2020). Points represent means, and error bars represent 95% confidence intervals. All scale responses range from 1 to 7. A three-item construct ( α = .82) was created, with greater perceived threat about COVID-19. In wave 1, Democrats ( M = 4.26, SD = 1.25) expressed greater perceived threat of COVID-19 than Republicans ( M = 3.90, SD = 1.39; t = -4.14, p < .001, d = -.40, 95% CI = [-.27, -.14]). Democrats’ responses showed an increasing time trend ( β = .08, SE = .04, p = .033), indicating they became increasingly concerned about the threat posed by the virus over time. Among Republicans, there was no significant time trend ( β = -.01, SE = .04, p = .83). These trends were significantly different from each other ( β = -.09, SE = .02, p < .001). While our data does not render causal claims, it is possible that the divergence in COVID-19 threat perceptions over time among Republicans and Democrats contributes to the divergence in vaccine attitudes between these groups over time. We revisit this proposition in the General Discussion. Our analyses revealed an interaction with political affiliation strength among Democrats—participants who identified as more strongly Democrat had a more positive time trend ( β = .03, SE = .01, p = .019), suggesting an increasing threat perception over time. This interaction was not significant for Republicans ( β = .01, SE = .02, p = .61). Trust in broad institutions The measures of trust in media, local government, and federal government were not highly correlated ( α = .66), and were therefore analyzed separately. Trust in media ( Fig 3 , Panel A) . In wave 1, Democrats ( M = 3.61, SD = 1.66) reported greater trust in the media than Republicans ( M = 2.73, SD = 1.65; t = -8.12, p < .001, d = -.53, 95% CI = [-.66, -.39]). There was no significant time trend for either Democrats ( β = .02, SE = .04, p = .57) or Republicans ( β = -.05, SE = .04, p = .20). However, the trend for Republicans was significantly more negative ( β = -.07, SE = .02, p < .001). The different trends we observe for Democrats and Republicans with respect to trust in the media may explain the divergence in perceived threat and vaccine attitudes between these groups over time (see General discussion ). Fig 3 Trust in broad institutions by political affiliation (March–August 2020). Points represent means, and error bars represent 95% confidence intervals. All scale responses range from 1 to 7. Trust in local government ( Fig 3 , Panel B) . In wave 1, Democrats ( M = 4.07, SD = 1.60) indicated lower trust in local government than Republicans ( M = 4.28, SD = 1.60; t = 2.01, p = .045, d = .13, 95% CI = [.003,.26]). Among Democrats, there was no significant time trend ( β = -.06, SE = .04, p = .18), though among Republicans, there was a decreasing time trend ( β = -.11, SE = .05, p = .015). These trends were significantly different from each other ( β = -.06, SE = .02, p = .004). Trust in federal government ( Fig 3 , Panel C) . In wave 1, Democrats ( M = 2.96, SD = 1.67) expressed lower trust in the federal government than Republicans ( M = 4.08, SD = 1.60; t = 10.52, p < .001, d = .68, 95% CI = [.55,.82]). Both Democrats and Republicans had decreasing time trends (Democrats: β = -.08, SE = .04, p = .036; Republicans: β = -.10, SE = .04, p = .025). These trends were not significantly different from each other ( β = -.02, SE = .02, p = .37). Attrition To rule out differential attrition, we tested whether the composition of our sample (i.e., age, gender, and political party) changed over time (see S1 Table ). Specifically, we tested whether participants who responded to waves 2–6 were significantly different at baseline (wave 1) from the full sample at baseline. The only significant change detected (Ps < .05) was with respect to participants’ age, though the differences were small—the average age was 38.5 at baseline, and remained between 39.9 and 40.8 at baseline among participants who responded to subsequent waves. We found no other systematic pattern of attrition among our participants. General discussion Over the course of six months of the COVID-19 pandemic, beginning with a relatively early phase prior to any U.S. directives to stay home (March 2020) and continuing through a cumulation of over 5 million cases (August 2020), we found a decrease in pro-vaccine attitudes and COVID-19 vaccination intentionsterm·def, as well as reduced intentions to get the influenza vaccine. These findings are contrary to our prediction that increased salience of COVID-19 would improve attitudes toward vaccines. Our analyses identify political ideology as the best predictor of the decreasing time trend across our three vaccine-related attitudes and intentions measures. In particular, we found that while Democrats’ responses remained fairly stable over time, Republicans shifted away from their lower initial responses and from Democrats’ responses, leading to increased polarization throughout the six-month period. Contrary to the polarization observed in our data, social and behavioral scientists have long argued that groups facing threats often come together, demonstrating stronger social cohesion [ 36 ], and more cooperative behaviors [ 37 , 38 ]. Researchers have also found that individuals’ sense of shared identity plays a role in promoting cooperative behavior in response to threat [ 39 – 41 ]. Considering our results in the context of these findings might suggest that our respondents’ sense of shared identity was dominated by their political ideology, as opposed to a broader (e.g. American) identity. What might be going on? Although the nature of our data does not render causal claims, it highlights potential explanations. First, we note that participants’ ratings of perceived COVID-19 threatterm·def followed a similar diverging pattern by party affiliationterm·def to our three vaccine-related measures during the study period. Democrats perceived COVID-19 threatterm·def to be greater at the start of the study than Republicans did, and this gap widened significantly as the study progressed. This trend is consistent with previous research showing that vaccine hesitancyterm·dom is related to perceived risk of a threat; when a VPD threat level is low, individuals are less motivated to take preventative action (i.e., immunize; for a review, see [ 42 ]). Our data offers one potential explanation for the polarization of threat perception: Republican and Democratic participants in our study reported consuming different sources of information. The most commonly checked news source for Republicans was Fox News (Republicans: 50%, Democrats: 8%; χ 2 = 164.55, p < .001) and for Democrats was CNN (Democrats: 47%, Republicans: 23%, χ 2 = 43.08, p < .001, see S6 Table ). Corroborating this proposition, a Pew Research Center poll conducted in March 2020 found that 56% of respondents whose main news source is Fox News believed that “the news media have greatly exaggerated the risks about the Coronavirus outbreak,” whereas this was only true for 25% of those whose main news source is CNN [ 43 ]. Of note, Facebook and Instagram, were also in the top four most consumed news sources for participants affiliated with either party. Extant work describes these platforms as echo chambers [ 44 , 45 ], which may exacerbate partisan exposure to news and information. Another trend highlighted by our data shows that similar to vaccine attitudes, Republicans’ trust in the media decreased significantly more during our study than Democrats’, suggesting these patterns might be related. According to Dr. Heidi Larson, an expert on vaccine hesitancyterm·dom and founder of the Vaccine Confidence Project, misinformation regarding vaccinations is more likely to take root when individuals do not trust the information source [ 46 ]. Future research might further examine the role of trust in the media on vaccine attitudes. While trust in media or media exposure may be driving COVID-19 threat perceptions and vaccine attitudes, there are many other possible explanatory factors that are not captured by our data or analyses. For example, it is possible that threat perceptions were influenced by how a respondents’ county or state was affected by COVID-19; up until June 2020, COVID-19 cases were more common in Democrat-leaning states [ 47 ], which might have amplified its salience early on and influenced attitudes and behavior. Further, although we included individual-level fixed-effects which control for all time invariant participant characteristics, and controlled for different trends by age and SES, we cannot rule out the possibility that other factors (e.g., educational attainment or population density) may have influenced the observed trends. Finally, as our data collection began after the onset of COVID-19, it is possible that the trend we observe for Republicans represents a return to a pre-pandemic baseline of vaccine-related attitudes. Contributions This work advances our understanding of how health-related attitudes evolve over time. Our focus on vaccine-related attitudes and intentions is important because experts agree that having enough people vaccinate against COVID-19 is key to stemming the pandemic [ 48 ]. More broadly, negative attitudes toward vaccination in general, and reduced vaccine uptake, is increasingly a public health concern [ 49 ]. This research provides insight into the trends of such vaccine hesitancyterm·dom, underlining the importance of risk salience and its roots in ideology and media exposure. This work also contributes to our understanding of political parties and polarization. Numerous anecdotes and reports have demonstrated a partisan divide in Americans’ response to the COVID-19 pandemic. For example, research found greater negative affective responses to wearing a face covering among politically right (vs. left) leaning individuals [ 50 ]. Here, we show that although these observations are valid, the reality is more nuanced. For example, our analyses reveal that polarization on vaccine measures—both attitudes and intentions—is driven primarily by self-identified Republicans’ gradual movement away from their initial responses whereas Democrats’ responses remained largely stable. This insight has important practical implications: It informs us about the dynamics of individuals’ attitudes, bringing us closer to understanding the underlying factors that influence attitudes and behaviors. Equipped with this knowledge, one could design more effective communications and interventions. Note on methodology and data availability The present study contributes to a small but growing literature in the social sciences using longitudinal data [ 51 ]. Using a longitudinal methodology allowed us to track individual-level changes over time. Merely observing a single point in time would allow us to observe across-group differences, but would lack the bigger picture of how polarization between these groups evolved. Another key advantage of panel data is that it allows us to include individual-level fixed effects, which control for the impact of omitted or unobserved time-invariant variables. Finally, panel data allows for more accurate inference of model parameters [ 52 ]. While the focus of this paper is vaccine attitudes, our broad dataset offers a unique opportunity to understand attitudes and behavior over time. Due to the richness of our data, its unique nature, and its timeliness, we believe it is important to make it available to other researchers interested in exploring it and publishing additional findings. The complete dataset is available at https://osf.io/kgvdy/ (see S2 and S3 Tables for all items collected). Supporting information S1 Appendix Additional information about sample exclusions. (DOCX) Click here for additional data file. S2 Appendix Additional information about political party affiliationterm·def. (DOCX) Click here for additional data file. S1 Table Attrition table. (DOCX) Click here for additional data file. S2 Table Summary table of measures and constructs included in the text. (DOCX) Click here for additional data file. S3 Table Summary table of measures excluded from the text. (DOCX) Click here for additional data file. S4 Table Regression results. (DOCX) Click here for additional data file. S5 Table Outcome measures by political party affiliationterm·def. (DOCX) Click here for additional data file. S6 Table Summary of news sources. (DOCX) Click here for additional data file. Data Availability All data and code are publicly available on the Open Science Framework at https://osf.io/kgvdy/ . Funding Statement UC San Diego Global Health Initiative (GHI): awarded to all authors; Project number: 1001288. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. https://medschool.ucsd.edu/som/medicine/divisions/idgph/research/Global-Health/grant-recipients/2019-2020/Pages/Faculty-Postdoc-Travel-and-Research.aspx .