When do persuasive messages on vaccine safety steer 1 COVID-19 vaccine acceptance and recommendations? Behavioural 2 insights from a randomised controlled experiment in Malaysia

Introduction. Vaccine safety is a primary concern among vaccine hesitant individuals. We examined how seven persuasive messages with different frames, all focusing on vaccine safety, influenced Malaysians to accept the COVID-19 vaccine, and recommend it to individuals with 59 different health and age profiles; i.e. healthy adults, elderly, and people with pre-existing health conditions. Methods. A randomized controlled experiment was conducted from the 29th of April to 7th of June 2021, which coincided with the early phases of the national vaccination programme when vaccine uptake data was largely unavailable. 5,784 Malaysians were randomly allocated into 14 experimental arms and exposed to one or two messages that promoted COVID-19 vaccination. Interventional messages were applied alone or in combination and compared against a control message. Outcome measures were assessed as intent to both take the vaccine and recommend it to healthy adults, elderly, and people with pre-existing health conditions, before and after message exposure. Changes in intent were modeled and we estimated the average marginal effects with respect to changes in the predicted probability of selecting a positive intent for all four outcomes. Results. We found that persuasive communication via several of the experimented messages improved recommendation intentions to people with pre-existing health conditions, with improvements ranging between 4 to 8 percentage points. In contrast, none of the messages neither significantly improved vaccination intentions, nor recommendations to healthy adults and the elderly. Instead, we found evidence suggestive of backfiring among this group with messages using negative frames, risky choice frames, and priming descriptive norms. Conclusion. Message frames that briefly communicate verbatim facts and stimulate rational thinking regarding vaccine safety may be ineffective at positively influencing vaccine hesitant individuals. Messages intended to promote recommendation of novel healthy interventions to people with pre-existing health conditions should incorporate safety dimensions