Electronic Nudges for Influenza Vaccination—Reply

In Reply We appreciate the Letter from Ms Wu and colleagues about our trial. Vaccination behavior is known to be influenced by multiple factors, and we completely agree that invitation timing, geographical accessibility to vaccination sites, and prior vaccination beliefs and experiences may be significant determinants. These factors were not directly incorporated into the interventions in our trial as we primarily focused on testing the delivery method and varying letter content across groups; however, owing to the randomized design, we would expect potential confounders such as geographical location and prior vaccination beliefs and experiences to be evenly distributed across groups. In addition, because all intervention letters in the trial were delivered on the same date (September 24, 2023), further analyses of the effects of differential invitation timing were not possible but should be investigated in future efforts. While Danish registries contain data across numerous domains, they do not contain detailed information about prior vaccination experiences or individual risk perception. We were therefore unable to include these data in our analyses. Interestingly, in another recent trial, offering free round-trip Lyft rides to vaccination sites in an attempt to decrease geographical barriers to vaccination did not result in increased vaccination rates.