Maternal Influenza Vaccination and Effect on Influenza Virus Infection in Young Infants

Abstract

It is recommended that pregnant women receive influenza vaccine because of their increased risk for influenza complications. Several prospective US-based observational studies concluded that maternal influenza vaccination delays the age at first infection and reduces severity of influenza illness; one randomized trial reported that vaccination reduced the incidence of laboratory-confirmed influenza. This nonrandomized, prospective, observational cohort study investigated the effect of seasonal influenza vaccination in pregnant women on the occurrence of influenza virus infection in infants to 6 months of age. Participants were 1160 mother-infant pairs with mothers who delivered one infant during 3 influenza seasons between 2002 and 2005 at the Navajo and White Mountain Apache Indian reservations. Serum specimens from the infants of women who were unvaccinated (n = 587) and vaccinated (n = 573) were collected and analyzed. The primary study outcome measures in infants were laboratory-confirmed influenza, with influenza-like illness (ILI), ILI hospitalization, and elevated influenza hemagglutinin inhibition antibody titers. Among the 1160 infants, 193 (17%) were hospitalized for ILI, 412 (36%) had only an outpatient ILI visit, and 555 (48%) had no ILI episodes. The ILI incidence rate was 7.2 per 1000 person-days for infants born to unvaccinated women and 6.7 per 1000 person-days for those born to vaccinated women. Compared with infants of unvaccinated women, there was a 41% reduction in the risk of laboratory-confirmed influenza virus infection (relative risk, 0.59; 95% confidence interval, 0.37–0.93) and a 39% reduction in the risk of hospitalization for ILI (relative risk, 0.61; 95% confidence interval, 0.45–0.84) among infants of vaccinated women. Moreover, hemagglutinin inhibition antibody titers to each of the 8 influenza virus antigens tested at birth and at 2 to 3 months of age were significantly higher in infants born to mothers vaccinated during pregnancy. These findings show that maternal influenza vaccination increased influenza antibody titers in infants through 2 to 3 months of age, reducing their risk of both influenza virus infection and ILI hospitalization during the first 6 months of life. These data demonstrate the public health importance of maternal influenza vaccination.