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.