Maternal Influenza Vaccination Associated with Flu Protection in Infants

Young infants born to mothers vaccinated against influenza while pregnant appear less likely to be infected with the flu or hospitalized for influenza-like illness (ILI) during the first 6 months of life, according to a study published in Archives of Pediatrics & Adolescent Medicine [2011; 165(2):104-111].

Although influenza activity and disease severity varies from season to season, studies have consistently identified young children to be at risk for influenza complications. Infants, however, are ineligible to be vaccinated until age 6 months. Influenza virus infection in infants is more prevalent among those aged 6 to 12 months who may be protected from the virus by maternal influenza antibodies acquired transplacentally or through breastfeeding. However, during severe influenza seasons, reports of morbidity and mortality rates among infants <6 months of age have exceeded those of older infants, the authors reported.

This study aimed to assess the effect of seasonal influenza vaccination during pregnancy on laboratory-confirmed influenza (LCI) in infants up to 6 months of age. Angelia A. Eick, PhD, formerly of Johns Hopkins Bloomberg School of Public Health, and now of the Armed Forces Health Surveillance Center, Silver Spring, Maryland, and colleagues conducted a nonrandomized, prospective, observational study on Navajo and White Mountain Apache Indian reservations, where children have significantly greater rates of respiratory infection, compared with the general US population. A total of 1169 women who delivered an infant during 1 of 3 influenza seasons from November 2002 to September 2005 were enrolled in this study. Of these, 1160 infant–mother pairs had at least 1 blood specimen collected and were included in this analysis. Mothers were eligible for study inclusion if they delivered a healthy infant at 36 weeks or later gestation during the enrollment periods. Eligible infants were aged ≤2 weeks at enrollment.

The women completed questionnaires about demographics, breast-feeding status, influenza risk factors, and vaccination status of all family members. The primary outcome measure, LCI, was defined as the first episode of ILI with (1) isolation of influenza virus from the nasopharyngeal aspirate specimen; (2) a 4-fold greater rise in hemagglutinin inhibition (HI) antibody serum collected at 2 to 3 months, compared with the previous serum specimen; or (3) a positive rapid influenza diagnostic test with a medical diagnosis of influenza. Of the 1160 infants, 908 episodes of ILI were confirmed; 193 children (17%) had an ILI hospitalization, 412 (36%) had only an outpatient ILI, and 555 (48%) had no ILI episodes. The ILI incidence rate per 1000 person-days for infants born to unvaccinated women was 7.2, compared with 6.7 for vaccinated women.

There was a 41% reduction in the risk of LCI virus infection (relative risk [RR], 0.59; 95% confidence interval [CI], 0.37-0.93) for infants of influenza-vaccinated mothers and a 39% reduction in the risk of ILI hospitalization (RR, 0.61; 95% CI, 0.45-0.84) for infants born to influenza-vaccinated women, compared with infants born to unvaccinated mothers. Among infants who had an ILI, those born to influenza-vaccinated mothers had a 42% reduction in the risk of the ILI requiring hospitalization (RR, 0.58; 95% CI, 0.41-0.83), compared with infants born to unvaccinated mothers. Furthermore, infants born to influenza-vaccinated women had significantly higher HI antibody titers at birth and at 2 to 3 months of age. The authors cited study limitations.

The researchers investigated mother–infant characteristics that could confound the findings and found no association with vaccination or outcome; however, there is always the possibility of uncontrolled residual confounding. Also, the study was conducted during 3 relatively mild influenza seasons. The results “underscore the public health importance of maternal influenza vaccination to prevent influenza in both pregnant women and their infants.” The 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,” the researchers concluded.