Repeated influenza vaccination of healthy children and adults: borrow now, pay later?

Abstract

A growing number of publications are recommending annual influenza vaccination of healthy children and adults. However, the long-term consequences of repeated influenza vaccination are unknown. We used a simple model of recurrent influenza infection to assess the likely impact of various repeated influenza vaccination scenarios. The model was based on a Markovian framework and was fitted on annual incidence rates of influenza infection by age. We found that natural influenza infection reduced the risk of being re-infected by 15.4% (95% confidence interval 7.1-23.0). Various scenarios of repeated influenza vaccination were then simulated and compared with a reference scenario where vaccination is given from age 65 years onwards. We show that repeated vaccination at a young age substantially increases the risk of influenza in older age, by a factor ranging between 1.2 (vaccination after 50 years) to 2.4 (vaccination from birth). These findings have important implications for influenza vaccination policies.

Fig. 1

Yearly influenza infection rates by age. Dots represent influenza infection rates by age during 18 epidemics and the blue solid line is their smoothed averages, calculated from data of the French Sentinel network (138 293 reported cases of influenza-like illness), in unvaccinated individuals between 0 and 65 years. The red solid line represents the corresponding infection rates obtained from the model best fitting these data, and the dashed red lines are the 95% confidence bandwidth.

Fig. 2

Relative risks of influenza infection in to different scenarios of repeated influenza vaccination. Relative risks are calculated as the ratio of influenza infection rates expected in individuals in the scenarios listed in the Table, to influenza infection rates in individuals vaccinated yearly from 65 years. (a) The calculations assume constant β and VE_P values. (b) Values for β and VE_P decrease linearly with age between 65 years and 100 years (from 0·17 to 0·05 and from 85% to 25% respectively).