Maternal flu vaccine protects infants

Maternal influenza vaccination was significantly associated with reduced risk of infant flu infection and hospitalization up to 6 months of age and increased influenza antibody titers in infants through 2 to 3 months of age, a study found.

Researchers conducted a nonrandomized, prospective, observational cohort study among 1,169 mother-infant pairs at hospitals located in Navajo and White Mountain Apache Indian reservations during three flu seasons (2002 to 2005). (Navajo and White Mountain Apache children have rates of acute respiratory infection that are significantly higher than in the general U.S. population. Because the flu vaccine is recommended for all pregnant women, a nonrandomized study was not possible.) Results were reported in the Oct. 4 Archives of Pediatrics and Adolescent Medicine.

Of the group, 49% of infants (n=573) were born to mothers who received flu vaccine during their pregnancy. The cohorts were otherwise similar except the vaccinated mothers were more likely to have a wood- or coal-burning stove in the house and to breastfeed.

Researchers measured laboratory-confirmed influenza, flu-like illness and hospitalization, and influenza hemagglutinin inhibition antibody titers. Influenza-like infections were defined as a medical visit with at least one of the following reported: fever of 38.0 °C or higher, diarrhea, or respiratory symptoms. Diarrhea was included because young infants can experience diarrhea with the flu.

Of the 1,160 infants, there were 908 flu-like infections: 193 children (17%) were hospitalized, 412 (36%) were treated as outpatients, and 555 (48%) had no medically attended flu-like infections. The flu-like infection incidence rate was 7.2 per 1,000 person-days and 6.7 per 1,000 person-days for infants born to unvaccinated and vaccinated women, respectively.

Fifty-two percent (n=605) of infants had a flu-like infection, of which 14% (n=83) were laboratory-confirmed influenza. The mean infant age at first flu-like infection was 47 days (median, 41; range, 0 to 175) and did not differ between infants born to vaccinated and unvaccinated mothers.

There was a 41% reduction in the risk of laboratory-confirmed influenza virus infection (relative risk, 0.59; 95% CI, 0.37 to 0.93) and a 39% reduction in the risk of flu-like infection hospitalization (relative risk, 0.61; 95% CI, 0.45 to 0.84) for infants born to influenza-vaccinated women compared with infants born to unvaccinated mothers. Infants born to influenza-vaccinated women had significantly higher hemagglutinin inhibition antibody titers at birth and at 2 to 3 months of age than infants of unvaccinated mothers for all 8 influenza virus strains investigated.

Limitations to the study include uncontrolled residual confounding despite studying variables that could have an influence, and that the study was conducted during three relatively mild flu seasons.

The researchers wrote, “These findings provide support for the added benefit of protecting infants from influenza virus infection up to 6 months, the period when infants are not eligible for influenza vaccination but are at highest risk of severe influenza illness. These findings are particularly relevant with the emergence of 2009 pandemic influenza A (H1N1) virus, which had a substantial effect on pregnant women and high hospitalization rates among young infants.”