In recent studies of influenza vaccine effectiveness (VE), lower effectiveness with increasing time since vaccination was observed, raising the question of optimal vaccination timing. We sought to evaluate the estimated number of influenza-associated hospitalizations among older adults due to potential changes in vaccination timing.
Using empirical data and a health state transition model, we estimated change in influenza-associated hospitalizations predicted to occur among the US population aged ≥65 years if vaccination were delayed until October 1. We assumed the vaccination timing, coverage, and effectiveness observed in 2012-2013 as a prototypical influenza season, approximately 7% monthly waning of VE, and that between 0% and 50% of individuals who usually get vaccinated earlier than October failed to get vaccinated. We also assessed change in influenza-associated hospitalizations if vaccination uptake shifted substantially toward August and September.
In a typical season, delaying vaccination until October increased influenza hospitalizations if more than 14% of older adults usually vaccinated in August and September failed to get vaccinated. The consequences of delayed vaccination depended heavily on influenza season timing, rate of waning, and overall VE. A shift toward vaccination in August and September led to, on average, an increase in influenza-associated hospitalizations, but this result was also sensitive to influenza season timing.
Consequences of delayed vaccination varied widely. Uncertainties about vaccine waning and effects of a delay on vaccine coverage suggest it is premature to change current vaccine recommendations, although it may be prudent to prevent a substantial shift toward early vaccination.