Table 1.

PICOS: inclusion and exclusion criteria.

PICOS Criteria	Inclusion Criteria	Exclusion Criteria
P: participants	The general population (at least one country).	Studies specifically on specific subgroups (e.g., healthcare professionals or people with a certain disease, such as diabetes or asthma) were excluded.
I: intervention	Questionnaire-based studies, i.e., administration of a questionnaire/survey to collect participants’ opinion/perception about COVID-19 vaccine hesitancy/acceptance.	Studies about other topics and/or that were not questionnaire-based.
C: comparison	When applicable (e.g., studies carried out in more than one country).
O: outcomes	To estimate the predictors of vaccine acceptance or vaccine hesitancy through a multivariate regression model.	Studies not estimating the predictors of vaccine acceptance or vaccine hesitancy through a multivariate regression model.
S: study design	Descriptive national studies, or descriptive studies enrolling at least 500 participants from the general population and carried out at a national level.	Studies enrolling less than 500 participants from the general population.