Effect of a nationwide intervention of electronic letters with behavioural nudges on influenza vaccination in older adults in Denmark

Before the 2022–23 influenza season, all eligible Danish citizens aged 65 years or older were randomly assigned to receive either usual care (no electronic letter) or one of nine letters that included a behavioural nudge: (1) standard electronic letter with person's name; (2) standard electronic letter repeated at 14 days; (3) depersonalised electronic letter without name; (4) gain-framed electronic letter focused on protection of self and loved ones; (5) loss-framed electronic letter focused on risk to self and loved ones; (6) collective-goal electronic letter focused on increasing vaccination rates together; (7) active-choice electronic letter including planning prompt; (8) gain-framed electronic letter focused on cardiovascular disease protection; or (9) electronic letter with expert-authority statement. Compared with usual care, Johansen and colleagues9 found that vaccination was higher in both the group receiving gain-framed letters highlighting cardiovascular disease protection (81·00% vs 80·12%; difference 0·89 percentage points [99·55% CI 0·29–1·48; relative risk [RR] 1·011 [99.55% CI 1·004–1·019]), and those receiving repeated standard letters (80·85% vs 80·12%; difference 0·73 percentage points [0·13–1·34]; RR 1·009 [1·002–1·017]). [...]Denmark is relatively unique in having an electronic letter system used nationally including for an older population, as well as high baseline vaccination rates, as indicated by the usual-care population having a vaccination rate of 80%, which could affect generalisability.