Understanding pediatric influenza virus immunity

August 1, 2008

A discussion of influenza vaccines, inactivated and live cold adapted attenuated virus; influenza virus and the immune response in children; and the differences in immune response elicited by both types of vaccine.

While the majority of pediatric illness elicited by influenza virus is self-limited, a small but significant number of fatal cases occur in children. For example, 153 fatal influenza-related cases were reported in children in the United States during the 2003-04 season. Though subsequent influenza seasons have been milder, it is likely that more pediatric deaths were due to influenza during that year in the United States than to any other vaccine-preventable disease.7

Vaccines against influenza

There are two types of influenza vaccines: inactivated virus, given by intramuscular injection, and a live cold-adapted attenuated virus given intranasally. The efficacy of influenza vaccine (VE) in children has been examined in multiple studies. A pooled analysis of several trials of the inactivated influenza vaccine showed a composite efficacy of 63% (95% CI, 45% to 70%).10 Recent studies examining the efficacy of the live virus vaccine in young children suggest an even better rate of protection, with a VE of 94% after two doses.11

Infants and vaccine

In terms of side effects, influenza vaccines are generally safe and well tolerated. One important concern regarding the live virus vaccine is a potential association with wheezing in infants under 12 months of age and in children with a history of recurrent wheezing.12,13 For now, use of the live vaccine is approved only for children ages 2 years or older who have no underlying high-risk conditions, including asthma.14

Although infants aged <6 months have the highest rates of influenza-related morbidity and mortality among children, influenza vaccine is not approved for this age group. Studies allow us to infer that infants receive some protection from influenza due to placental transfer of maternal antibodies, and raise the potential benefit of vaccinating pregnant women against the virus.15 Additional protection of infants against influenza may be achieved by immunizing all other household members and childcare providers.16