The previous literature has already demonstrated that it is necessary to evaluate the efficacy and possible side effects of each of the many vaccines against COVID-19 separately (27). In our case, given the difference observed between the levels of intention to be vaccinated with each vaccine, prediction models were tested for each one separately. There was no effect of the level of education on the intention to be vaccinated. Still, the results showed that there is a greater intention to be vaccinated using the Oxford-AstraZeneca vaccine. This result sets a precedent for future research, interested in evaluating the will to be immunized against COVID-19 and considering separately the vaccines offered in each country.
In addition to the more favorable intention to the Oxford-AstraZeneca vaccine, the analyses showed that each vaccine's intention also has some different predictive factors. For example, smaller Vaccine Uncertainty Index and greater Fear of COVID-19, in this order, were associated with a greater intention to get this vaccine. These variables explained up to 26.6% of the will to take the Oxford-AstraZeneca vaccine. In contrast, sociodemographic variables, such as sex, age, having a previous diagnosis of COVID-19, or a family member or close friend with COVID-19, and political orientation, had no significant effect on the model.
Concerning the Sinovac-CoronaVac vaccine, Vaccine Uncertainty Index, political orientation, and Fear of Covid-19, in this order, explained up to 44.3% of the intention to use it. More specifically, smaller uncertainty, a left-wing politics identification, and greater fear of COVID-19 predict greater intention to undergo vaccination. At the same time, the other variables did not have any impact.
Only the Vaccine Uncertainty Index explains a robust portion of the model variance (approximately 25% for the intention to be vaccinated with Oxford-AstraZeneca and 43% with Sinovac-CoronaVac), even though the role of Fear of COVID-19 is also significant in both models. The belief that SARS-CoV-2 vaccines are unsafe has already been considered one of the main causes of vaccine hesitancy (28). Also in the case of influenza, aspects such as disbelief in vaccine efficacy and fear of side effects are factors associated with older adults' hesitation to get vaccinated (29). In addition to these findings, our result suggests that insecurity about unknown side effects, and about the “record time” of this vaccine development, may be risk factors for achieving the mass immunization.
The Fear of Covid-19 also significantly affected both models, although its contribution was relatively small. Others also support this finding regarding the role of perceived vulnerability to prophylactic behavior (30; 31). In sum, fear appears to have an important adaptive role (32-34) that, in this pandemic context, can contribute to the vaccine intention.
One of the differences between the models is that although Vaccine Uncertainty Index's role is relevant in both, its contribution is more significant in predicting the intention to vaccinate with Sinovac-CoronaVac. This could be explained given the greater variability in responses to the intention directed to this vaccine, which impacted the associations' strength. Still, Sinovac-CoronaVac was the only vaccine whose intention to use had some impact from the political orientation. This effect, although small, should not be disregarded. Unlike other world political leaders, Jair Bolsonaro (president of Brazil) tends to endorse measures that go against international health agencies' recommendations (35). Sinovac-CoronaVac, for example, was, at a certain time, target of his criticism. Therefore, it makes sense that those respondents whose political orientation is closer to that of the president (extreme right) also have a less favorable attitude towards the use of Sinovac-CoronaVac.
Still concerning Sinovac-CoronaVac vaccine and the effects of political orientation, the relationship between the variables shows that less Vaccine Uncertainty Index is associated with a greater vaccination intention. However, this relationship is more robust in the more right-wing political orientation spectrum. Interestingly, when uncertainty is low, there is a high intention to be vaccinated, regardless of political orientation. In contrast, when uncertainty is high, differences in intention as a function of political orientation become more evident. This may suggest that information (reduction of uncertainty) may be an effective strategy to overcome political orientation's effect.
These results present contributions to the vaccination campaigns against COVID-19, mainly with a focus on elderly populations. First, campaigns should focus on making vaccine safety clear to reduce uncertainty about them, especially concerning their development process and the scientific rigor of that process. For example, in the case of vaccines available in Brazil, both have already been shown to have a high level of safety (36; 37), although Sinovac-CoronaVac has not yet demonstrated robust data related to the elderly population (38).
In this context, health professionals and science communicators, who will have a very relevant role in this campaign, must be very transparent in their communications, avoiding dubious information (39). Also, another important factor is the effect that the news's focus can trigger on the spectator (40). For example, although the record time with which it was possible to develop the vaccine is a scientific milestone, this factor seems to arouse a negative attitude in the general population. Therefore, instead of focusing on the “speed of development of vaccines”, the media should clarify how all the steps have been taken to ensure a safe and efficient vaccine. In other words, it is necessary to make clear the nature of the scientific method itself to reduce misinformation and the cognitive biases that predispose to denialism and vaccine hesitancy (41).
The positive impact of fear, although smaller, is also relevant and should be considered in the planning of public campaigns aimed to educate the population to get vaccinated against COVID-19 (42). For example, some authors suggest that vaccination campaigns focusing on the older population should focus on making the risk of complications more salient (43). Basically, in moments like this pandemic, it is necessary for the authorities to incentive the "optimistic anxiety", described as a phenomenon in which "Citizens must be anxious enough to take the advice from the authorities to heart and optimistic enough as to feel that their actions make a difference – and therefore they will carry them out" (44).
Concerning political orientation, given its specific effect on the intention to vaccinate with Sinovac-CoronaVac, it must be taken into account by public agencies responsible for vaccination campaigns, since it can be a variable that negatively impacts the adherence of a portion of the population. Vaccination campaigns need to be developed focusing on individuals of the right-wing political spectrum to overcome their resistance. Still, given the positive effect that celebrity endorsement has on forming a more favorable attitude towards vaccines (45), it would be crucial for campaigns to select celebrities who is favorable to the vaccine, but with a neutral or even right-wing political orientation.
This study has some limitations. The first one regarding the size and characteristics of the sample. Despite contemplating different educational levels and income, there is a huge concentration in the privileged classes, which do not necessarily represent the reality of the Brazilian elderly. Still, the fact that the research was carried out through the internet already causes a sample bias, as it leaves out participants who do not have access to this technology (46). However, this method was necessary, given the pandemic context, in which face-to-face participation could increase the respondents' risk of contamination. Also, due to the participant's age and the absence of face-to-face support to assist the respondent, the form had to be reduced, including the fewest questions possible. As a result, it was not possible to investigate other variables' role, which could also predict the intention to receive the vaccine, such as the tendency to nurture conspiracy beliefs and the level of credibility toward science. Therefore, these are variables that will need to be considered in future investigations.
In summary, this study contributes to public campaigns focusing on the mass immunization of the elderly against COVID-19 by emphasizing the importance of understanding the specificity of each vaccine separately. Also, it highlights the need to consider promotion aspects of a more favorable attitude towards vaccination, such as the positive effect of fear of COVID-19, and risk aspects for vaccination, such as the negative effect of vaccine uncertainty on the intention to vaccinate against this pandemic disease.