Prevalence and Determinants of Hesitancy
A total of 11,860 vaccinated and 10,122 unvaccinated participants across eight countries were recruited between 8 April and 2 July 2021. The prevalence of COVID-19 vaccine hesitancy disaggregated by participants’ gender is presented in Figure 1 (for further detail, see Figures S5-S7 & S11-S14). These estimates are based on the full sample of 21,982 respondents, i.e. including those who had already received one or two vaccine shots and who were coded as not hesitant, and were screened out of the survey after collecting their key sociodemographic characteristics.
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The estimates point to considerable heterogeneity across countries. COVID-19 vaccine hesitancy was lowest in Spain, with 5.86% (95% CI 0.05-0.07) of women and 5.19% (95% CI 0.04-0.06) of men hesitant to receive the COVID-19 vaccine, compared to Bulgaria, showing the highest rates with 55.16% (95% CI 0.52-0.59) of women and 46.87% (95% CI 0.43-0.50) of men hesitant. Across countries, men were less skeptical of the COVID-19 vaccine compared to women.
Unvaccinated participants who indicated that they would only intend to get vaccinated with vaccines of a certain type were asked to specify which vaccines they would accept (see Figure 2 and Figures S8-S10). Across countries, vaccine-specific willingness to get vaccinated was highest for the BioNTech/Pfizer vaccine, ranging from 57.09% (95% CI 0.51-0.63) of respondents in Poland, to 93.17% (95% CI 0.92-0.95) in Germany. While overall acceptance of the viral vector vaccines was consistently lower than it was for the mRNA vaccines, we did observe substantial differences in the perceptions surrounding the AstraZeneca vaccine. Notably, 31.92% (95% CI 0.26-0.38) of the conditionally willing respondents in the UK indicated that they would accept this vaccine, compared to only 2.6% (95% CI 0.02-0.04) in Germany.
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A summary of the sociodemographic characteristics of the unvaccinated participants who completed the full survey and participated in the survey experiment is provided in Table S1, including a breakdown by target country. To understand the sample composition further, we provide additional information about the COVID-19 vaccination rate in each target country by age group at the time of data collection (see supplementary Table S2) as well as census information on gender, age, and education (see Table S3).
Findings from multivariate logistic regression analyses assessing demographic factors associated with vaccine hesitancy are summarised in Table S4. With the exception of Spain, Sweden, and Poland, women were significantly more hesitant towards COVID-19 vaccines than men. In most countries, vaccine hesitancy was significantly higher in older age groups (ranging from OR=1.12, 59% CI 1.02- 1.24, p<0.01 in Poland to OR=1.67, 95% CI 1.48 - 1.89, p<0.001 in Sweden and the UK). The trend was reversed in Germany, with a significant decrease in vaccine hesitancy among older age groups (OR=0.80, 95% CI 0.75 - 0.85, p<0.001). These differences are most likely a result of age-based priority access to vaccinations as well as variation in the timing of our surveys. Specifically, while the German survey was launched earlier and the sample of unvaccinated participants is thus less biased (given that only around 25% of the population had received a COVID-19 vaccine by then, see Table S2), the samples of unvaccinated participants in other countries may over-represent vaccine-hesitant individuals in the older age groups as those willing had likely already received their vaccination at that point in time. This explanation is further supported by the differential representation of older participants in Germany compared to the other countries (above-65-year-old participants account for around 16% of the German sample, but for less than 2% in all other countries’ samples).
In some countries, vaccine hesitancy was significantly associated with lower education. In Germany, for instance, the predicted probability of refusing the COVID-19 vaccine was 45.07% (95% CI 39.95-50.20 %, p<0.001) for participants who had not completed secondary education, compared to 15.89% (95% CI 12.98-18.79 %, p<0.001) for participants who held a university degree, all else equal. Participants who were employed reported significantly lower levels of COVID-19 vaccine hesitancy in Spain and in Sweden (OR= 0.56, 95% CI 0.39 – 0.80, p<0.001, OR= 0.62, 95% CI 0.45-0.86, p<0.001, respectively), whereas respondents’ employment status did not significantly affect vaccine hesitancy in any other country.
Participants' motives and reasons for their vaccine hesitancy, elicited from their free-text statements, can be grouped into five themes (see Table S5). Fear of side effects constituted the most common theme, mentioned by 22% of vaccine-hesitant participants in Spain and up to 41% of vaccine-hesitant participants in Italy. While many participants listed side effects as a general concern, some responses offered stronger sentiments, referring to COVID-19 vaccines as “poison” and revealing a fear of experiencing lethal side effects. Some respondents were concerned about side effects linked to (1) pre-existing medical conditions such as chronic diseases or allergies, (2) potential infertility or harm to an (unborn) child, and (3) certain characteristics of specific vaccines, including concerns about “genetic modification” introduced by mRNA vaccines or blood clot incidents associated with the AstraZeneca vaccine.
The lack of evidence regarding the long-term effects of the COVID-19 vaccines was listed as another major concern spurring vaccine hesitancy, cited by 17% of participants in Bulgaria and up to 44% of participants in France. Several respondents portrayed the current vaccination campaign as a large-scale human experiment (“we are all just guinea pigs “, see Table S5) and expressed concerns about the speed with which the vaccines were developed. Relatedly, many respondents pointed to fears about detrimental middle- to long-term health impacts of the COVID-19 vaccines and the lack of scientific evidence on such long-term effects.
In addition, some participants cited low levels of trust in the quality and efficacy of COVID-19 vaccines as a key barrier to getting vaccinated (ranging from 6% of participants in the UK to 11% of participants in France), which was often linked to concerns about the vaccines’ potency against more recent variants of the coronavirus and uncertainty or concerns about whether and to what extent vaccinated individuals might still transmit the virus.
Another theme that emerged, listed by 7% of participants in Poland and by up to 14% of Swedish respondents, was related to the perception that COVID-19 does not represent a substantial health threat, thus rendering vaccination unnecessary. Here, many participants emphasised their own good health and argued they were confident that their own immune system would be sufficiently capable of fending off the virus. Others reported a low perceived risk of contracting the virus due to limited social interactions or little mobility, and referred to COVID-19 as simply “a flu” or even denied its existence altogether.
A final commonly cited barrier to vaccination was distrust towards the government, pharmaceutical companies or “elites” in general, listed by 3% of participants in Poland and up to 12% of participants in Spain. More specifically, participants repeatedly suggested that profits for pharmaceutical companies were the primary purpose of the vaccination campaign, viewed the vaccination as a means of state control, and voiced concerns or fears about being experimented on, while some participants sympathised with conspiracy theories surrounding the vaccines or COVID-19.
Causal Effects of Information Treatments
The outcomes of the survey experiment are presented separately for each country in Table 1 (and additionally in supplementary Figures S15-S22). In Germany, three out of four treatments significantly increased participants’ willingness to vaccinate. Specifically, the odds of accepting the COVID-19 vaccine were 1.5 times higher for participants who were presented with the COVID-19 risk reduction message, relative to participants in the control group (OR=1.46, 95% CI 1.09 –1.97, p<0.05). Starting from a baseline acceptance level of 27% in the control group, the treatment effect thus corresponds to a six-percentage-point increase in respondents’ intention to vaccinate. Messages highlighting hedonistic benefits and the benefits of owning a vaccination passport were also significantly associated with significantly higher odds of COVID-19 vaccination willingness: 1.43 (95% CI 1.06 – 1.93, p<0.05) and 1.45 (95% CI 1.09 – 1.95, p<0.05), respectively. Respondents exposed to the altruistic message were also somewhat more likely to indicate that they would accept a vaccination offer, but the difference to the control group was not statistically significant (OR=1.32, 95% CI 0.99 – 1.77, p=0.06). Due to budget restrictions, we excluded the altruistic message as the least effective treatment from the survey experiment in the seven remaining countries.
There was vast heterogeneity across countries with regard to the impact of the three experimental information treatments on respondents’ willingness to get vaccinated. In the UK, the vaccination certificate message significantly increased the odds of intending to get vaccinated by 1.51 (95% CI 1.02 – 2.24, p<0.05) compared to the control group, corresponding to an increase from 22% to 28% of participants being willing to get vaccinated. In Bulgaria, Poland, France, Italy, and Sweden, none of the information treatments significantly improved participants’ reported vaccination intentions. In fact, treatment effects even pointed in the opposite direction in some countries, thus revealing potential harmful effects of public messaging, though these effects were significant only in Spain. While informing participants about the risk reduction effects associated with COVID-19 vaccines was found to have positive effects in Germany, the same information treatment caused an adverse effect on vaccination intentions in Spain, leading to an eight-percentage-point drop in vaccination willingness relative to the control group (OR=0.66, 95% CI 0.46 – 0.96, p<0.05), albeit from the higher baseline rate of 67% in Spain.
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Heterogeneity in Treatment Effects
Heterogeneity in treatment effects both across and within countries was further assessed in a model-based recursive partitioning approach, including age, gender, education, employment and country of residence as explanatory variables (see Table 1). For the hedonistic benefits treatment, none of the subgroups identified by the algorithm based on a combination of these variables exhibited significant treatment effects. The corresponding regression tree shown in Figure S23 displayed only seven nodes, thus suggesting that there was, overall, relatively little heterogeneity with regards to the impact of the hedonism treatment. In contrast, there was evidence for substantial heterogeneity in the impact of the COVID-19 risk reduction and vaccination certificate treatments, as illustrated by a total number of 29 and 27 nodes in the regression trees, respectively (see Figures S24-S25). More specifically, two subgroups who received the risk reduction message revealed a significantly lower likelihood to get vaccinated against COVID-19, relative to the control group: Spanish participants who were not employed were less likely to accept the COVID-19 vaccine, relative to the control group (OR=0.49, 95% CI 0.25-0.93, p=0.030) (see Table 2). Participants who were Italian or German, female and not employed also had lower odds of accepting the vaccine (OR=0.38, 95% CI 0.17-0.83, p=0.015). Conversely, the risk reduction message had positive effects for the subgroup of Italian and German women who were employed and had a lower level of education (OR=2.23, 95% CI 1.02-4.90, p=0.045). With regard to the vaccine certificate message, there were two subgroups with positive effects: Italian, Swedish and German participants, aged 18-24 or 65+ years, with primary, secondary, or further education showed a substantial increase in the likelihood of vaccinating (OR=3.15, 95% CI 1.56-6.36, p=0.001); similarly, those with higher education and in the middle and higher age groups (OR=2.76, 95% CI 1.53-4.98 p=0.001).
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