There is limited data on the retention of participants on online crowdsourcing platforms. This presents a summary of retention within a study on vaccination attitudes with two time points separated by approximately 12 months on Prolific. Of the 331 participants who completed the initial survey in 2018, 262 remained active on the platform in 2019. Of these, 254 (76.74% of total baseline sample; 96.95% of active users) completed the follow-up survey, with a majority completing within 24 hours of the start of follow-up. Older participants and women were more likely to be retained. Overall, Prolific shows similar retention characteristics to conventional cohort studies with substantially reduced researcher burden.