Influence of a COVID-19 vaccine’s effectiveness and safety profile on vaccination acceptance

PNAS – Proceedings of the National Academy of Sciences of the United States of America
March 02, 2021; vol. 118 no. 9


Psychological and Cognitive Sciences
Influence of a COVID-19 vaccine’s effectiveness and safety profile on vaccination acceptance
Robert M. Kaplan and Arnold Milstein
PNAS March 9, 2021 118 (10) e2021726118;
Acceptance of vaccines has been on the decline in recent years. Despite encouraging early results for coronavirus vaccine trials, achieving herd immunity requires substantial uptake. We presented scenarios varying vaccine efficacy, minor side effects, and severe reactions to a sample representative of the US population. Vaccine acceptance improved when the efficacy increased beyond 70%. Respondents were unaffected by the probability of minor side effects, such as a sore arm or fever lasting 24 h. The chances of accepting the vaccine were lower when the probability of serious adverse reactions was 1/100,000 in contrast to 1/million or 1/100 million. A replication showed that the results were largely unchanged following the public announcement that the vaccines were 95% effective.
Although a safe and effective vaccine holds the greatest promise for resolving the COVID-19 pandemic, hesitancy to accept vaccines remains common. To explore vaccine acceptance decisions, we conducted a national survey of 1,000 people from all US states in August of 2020 and a replication in December of 2020. Using a 3 × 3 × 3 factorial experimental design, we estimated the impact of three factors: probability of 1) protection against COVID-19, 2) minor side effects, and 3) a serious adverse reactions. The outcome was respondents’ reported likelihood of receiving a vaccine for the coronavirus. Probability of vaccine efficacy (50%, 70%, or 90%) had the largest effect among the three factors. The probability of minor side effects (50%, 75%, 90%) including fever and sore arm, did not significantly influence likelihood of receiving the vaccine. The chances of a serious adverse reaction, such as temporary or permanent paralysis, had a small but significant effect. A serious adverse reaction rate of 1/100,000 was more likely to discourage vaccine use in comparison to rates of 1/million or 1/100 million. All interactions between the factors were nonsignificant. A replication following the announcement that vaccines were 95% effective showed small, but significant increases in the likelihood of taking a vaccine. The main effects and interactions in the model remained unchanged. Expected benefit was more influential in respondents’ decision making than expected side effects. The absence of interaction effects suggests that respondents consider the side effects and benefits independently.