Post-intervention outcome measures
Immediately after the intervention, participants responded to items measuring outcomes. The primary outcome measure was intention to receive a COVID-19 vaccine booster dose. This was assessed with a single item (‘How likely is it that you will get a booster dose of COVID-19 vaccine?’, with 5 response options: definitely, probably, I’m not sure, probably not, definitely not), consistent with survey questions used in previous research [Secondary outcome measures were beliefs about COVID-19 vaccine booster doses. These were assessed by asking participants to indicate their agreement with belief statements about COVID-19 vaccine booster dose safety, effectiveness, necessity for protecting one’s own health, necessity for protecting others’ health, and risks associated with not vaccinating (response options were a 5-point scale: strongly disagree, slightly disagree, neither agree nor disagree, slightly agree, strongly agree. See 