Public misperceptions of COVID-19 vaccine effectiveness and waning: experimental evidence from Ireland.

The study set out to measure public understanding of COVID-19 vaccine effectiveness (VE) and how effectiveness wanes with time since vaccination. Because perceived VE is a strong predictor of vaccine uptake, measuring perceptions can inform public health policy and communications. Online randomised experiment. The study was undertaken in Ireland, which has high vaccination rates. A nationally representative sample (n = 2000) responded to a scenario designed to measure perceptions of COVID-19 VE against mortality. The length of time since vaccination in the scenario was randomly varied across four treatment arms (2 weeks, 3 months, 6 months, and 9 months). The public underestimates VE, with substantial variation in perceptions. A majority (57%) gave responses implying perceived VE against mortality of 0-85%, i.e., below scientific estimates. Among this group, mean perceived VE was just 49%. Over a quarter (26%) gave responses implying perceived VE greater than 95%, i.e., above scientific estimates. Comparing the four treatment groups, responses took no account of vaccine waning. Perceived VE was actually higher 9 months after vaccination than 2 weeks after vaccination. Despite high vaccination rates, most of the public in Ireland underestimates VE. Furthermore, the general public has not absorbed the concept of vaccine waning in the months following vaccination. Both misperceptions may reduce vaccine uptake, unless public health authorities act to correct them through improved communication.

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