Across the 24 test conditions, Americans’ willingness to be vaccinated varies from a low of 36.0% of adults to a high of 51.8%. If a vaccine were released this year and only met the minimum FDA efficacy threshold of 50%, 36.0% of adults say they would get it. The acceptance rate increases if the vaccine were released in early 2021 (between January and March) and with more reassuring information about the approval process, side effects and efficacy.

Using regression modeling, we isolated each piece of information to measure its effect on public acceptance of a vaccine. While respondents were randomly assigned to the different experiment groups, there were still differences across these groups in demographic characteristics and in whether they got the 2019 flu vaccine, which is closely related to expected compliance with the COVID-19 vaccine. To account for this, we ran three regression models. One reports the simple mean among groups, the second adjusts for demographic controls (e.g., age, gender, education level, race/ethnicity and political party), and the third adds additional controls for whether the respondent got the 2019 flu vaccine and other COVID-related attitudes captured in the survey.

The study finds that vaccine efficacy and release timing are significantly related to public acceptance across all models, while the absence of side effects is significantly related in just the final model that includes all weighting adjustments. Whether FDA approval following three rounds of clinical trials was mentioned, or just FDA approval, did not affect acceptance.