The misunderstanding of vaccine efficacy.

Although the efficacies of vaccines against SARS-CoV-2, i.e., the virus that causes Covid-19, have been publicized and praised, and although they are assumed to encourage vaccine compliance, little is known about how well these figures are understood by the general public. Our study aims to fill this gap by investigating whether laypeople have an adequate grasp of what vaccine efficacy means and, if not, which misconceptions and consequences are the most common. To this end, we carried out three online behavioral experiments involving 1800 participants overall. The first, exploratory experiment, with a sample of 600 UK participants, allowed us to document, by means of both an open-ended question and a multiple-choice question, a common misinterpretation of the efficacy of SARS-CoV-2 vaccines as the non-incidence rate among the vaccinated. We formally demonstrated that this error leads to a systematic overestimation of the probability of individuals who are vaccinated developing Covid-19. The second experiment confirmed the prevalence of this misinterpretation in a new sample of 600 UK and Italian participants, by means of a slightly different multiple-choice question that included more response options. Finally, in a third experiment, involving another 600 UK and Italian participants, we investigated the behavioral implications of the documented error and showed that it might undermine the general positive attitude toward vaccines as well as the intention to get vaccinated. On the whole, the results of this study reveal a general misunderstanding of vaccine efficacy that may have serious consequences for the perceived benefits of SARS-CoV-2 vaccines and, thus, the willingness to be vaccinated.