Repeated influenza vaccination at a young age substantially increases the risk of influenza at older ages.
Carrat F, Lavenu A, Cauchemez S, Deleger S. Repeated influenza vaccination of healthy children and adults: borrow now, pay later? Epidemiology & Infection 2006;134(1):63-70.
This study shows that repeated influenza vaccination at younger ages may double the risk of influenza in the elderly. The study suggests that the “possible benefits of vaccinating children after 5 years of age, and otherwise healthy adults—particularly over a long period and mainly for economic reasons—could be outweighed by severe clinical consequences and increased costs in the elderly.” Moreover, the findings are “solely due to differences between vaccine-induced immunity and naturally acquired immunity.” Unlike vaccination, naturally acquired immunity can provide long-lasting protection against subsequent infection by the same viral subtype.