Cristina A. Godinho, Lucy Yardley, Afrodita Marcu, Fiona Mowbray, Emma Beard, Susan Michie

Vaccination is an effective preventive measure to reduce influenza transmission, especially important in a pandemic. Despite the messages encouraging vaccination during the last pandemic, uptake remained low (37.6% in clinical risk groups). This study investigated the effect of different types of messages regarding length, content type, and framing on vaccination intention. An online experiment was conducted in February 2015. A representative sample of 1424 people living in England read a mock newspaper article about a novel influenza pandemic before being randomised to one of four conditions: standard Department of Health (DoH) (long message) and three brief theory-based messages - an abridged version of the standard DoH and two messages additionally targeting pandemic influenza severity and vaccination benefits (framed as risk-reducing or health-enhancing, respectively). Intention to be vaccinated and potential mediators were measured. The shortened DoH message increased vaccination intention more than the longer one, by increasing perceived susceptibility, anticipated regret and perceived message personal relevance while lowering perceived costs, despite the longer one being rated as slightly more credible. Intention to be vaccinated was not improved by adding information on severity and benefits, and the health-enhancing message was not more effective than the risk-reducing. A briefer message resulted in greater intention to be vaccinated, whereas emphasising the severity of pandemic influenza and the benefits of vaccination did not. Future campaigns should consider using brief theoretically-based messages, targeting knowledge about influenza and precautionary measures, perceived susceptibility to pandemic influenza, and the perceived efficacy and reduced costs of vaccination.