Research in context
Evidence before this study
Vaccination has been shown to reduce influenza infection and its attendant complications. As such, seasonal influenza vaccination is strongly endorsed as a core public health strategy. However, global influenza vaccination rates remain suboptimal, including among older adults at high risk. Although countries worldwide implement various influenza vaccination campaigns, these communication strategies have been largely untested in rigorous nationwide implementation trials. We assessed the existing influenza vaccination implementation trial literature by searching PubMed on Dec 30, 2022, with no date or language restrictions, using the search terms (“implementation” OR “behavioural science” OR “nudges” OR “letters” OR “reminders” OR “messages”) AND “influenza” AND (“vaccination” OR “vaccine”) AND (“trial” OR “randomised”).
Added value of this study
In a first-of-its-kind nationwide, pragmatic, registry-based, cluster-randomised implementation trial, 964 870 Danish adults aged 65 years or older received usual correspondence or one of nine electronic letters designed on the basis of different behavioural concepts to encourage influenza vaccination. A letter emphasising potential cardiovascular benefits of vaccination and a reminder letter sent again at 14 days after initial delivery both modestly but significantly increased influenza vaccination rates. Effectiveness extended across various subgroups, including those without influenza vaccination in the preceding season. Effectiveness in nudging influenza vaccination was consistently observed across all five major Danish regions.
Implications of all the available evidence
In a country with a high background rate of influenza vaccination, certain behaviourally informed, electronic nudges delivered in advance of vaccine availability incrementally increased vaccine uptake (approximately 1% absolute increase). Although the magnitude of effectiveness of the successful nudging strategies might seem modest, the low-touch, inexpensive, and highly scalable nature of these electronic letters might have important population-level public health implications.