Background: Inactivated influenza vaccine is recommended for all pregnant women and children, except for infants younger than six months for whom the vaccine is poorly immunogenic. Maternal vaccination during pregnancy could possibly protect infants through the cross-placental transport of immunoglobulin G antibodies to the fetus, and through the transfer of immunoglobulin A to infants via breast milk. Benowitz and colleagues conducted a matched case-control study of infants to assess the effectiveness of influenza vaccine given to pregnant women in decreasing hospitalizations for influenza among their infants.

The Study: Infants younger than 12 months who were hospitalized for influenza that was confirmed by direct fluorescent antibody testing were eligible. Case infants were matched to control infants (i.e., infants hospitalized for reasons other than influenza). Mothers were considered to be vaccinated against influenza if they had written documentation of receiving the vaccine during pregnancy. Women vaccinated within 14 days of delivery were excluded. A matched odds ratio was calculated for mothers of case infants, compared with mothers of matched control infants.

Results: A total of 113 case infants were matched to 192 control infants. Among infants younger than six months, two of 91 case infants (2.2 percent) had mothers who were vaccinated during pregnancy, compared with 31 of 156 control infants (19.9 percent). Overall, vaccination during pregnancy was 90.7 percent effective in preventing influenza hospitalization among infants younger than six months (P = .001). This increased to 91.5 percent effectiveness (P = .001) after adjusting for potential confounders. No statistical benefit was seen among infants six to 12 months of age regarding hospitalization rates, although the study was not adequately powered to detect differences in this age range.

Conclusion: Inactivated influenza vaccine given to pregnant women is highly effective in preventing hospitalization from influenza among their infants who are younger than six months. This strategy can protect young infants at risk of influenza for whom no vaccine is currently available.