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Maternal influenza immunization appears to protect mothers, infants, according to new trial

Wednesday, October 15 2008 | Comments

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What's This? Infants born to women who received an influenza vaccine in the third trimester were 63% less likely than control infants to develop influenza in the first 6 months of life, suggests a study of mothers and infants in Bangladesh.

To assess the safety and clinical effectiveness of inactivated influenza immunization administered during pregnancy, researchers at Johns Hopkins University partnered with researchers in Bangladesh, recruiting participants for the current study from Bangladesh's Mother's Gift project, a randomized trial with the primary goal of assessing the effectiveness of sequential maternal and infant immunization with the pneumococcal vaccine. In the current study, mothers in the control group received pneumococcal polysaccharide vaccine only, and mothers in the treatment group received inactivated influenza vaccine only. In all, 340 mothers were randomized.

The primary outcome in infants was the first episode of laboratory-confirmed influenza before 24 weeks of age. Study outcomes for both mothers and infants were number of episodes of respiratory illness with fever or of documented fever >38 degrees C, clinic visits with respiratory illness, and episodes of diarrhea.

Among infants of mothers who received the influenza vaccine, there were fewer cases of laboratory-confirmed influenza than among infants in the control group (6 vs 16 cases), with a vaccine effectiveness of 63% (95% CI, 5%-85%). Respiratory illness with fever occurred in 110 infants in the influenza-vaccine group and 153 infants in the control group, with a vaccine effectiveness of 29% (95% CI, 7%-46%). In addition, among mothers who received the influenza vaccine, there was a 36% reduction in the rate of respiratory illness with fever (95% CI, 4%-57%).

The researchers noted that although the confidence limits for the effectiveness of passive immunization were wide, the point estimates of effectiveness against both laboratory-proven influenza and multiple clinical endpoints in the infants were similar to those reported in trials of active influenza vaccine in infants aged >6 months.

The researchers also acknowledged as a limitation the fact that control mothers received the pneumococcal vaccine rather than placebo. This design could have resulted in underestimating the true effect of maternal influenza immunization, since control infants may have had a reduced incidence of pneumococcus-associated respiratory illness with fever and a reduced number of clinic visits.

The investigators determined that influenza immunization of fewer than 16 mothers would prevent 1 laboratory-proven influenza illness in the infants.

"Our study suggests that the antenatal-immunization strategy should be evaluated further for the prevention of influenza," the authors concluded, adding that researchers will need to carry out large studies with follow-up periods of several years to show the full effect of the strategy of maternal influenza immunization, particularly in tropical and subtropical areas. (Zaman K, et al. N Engl J Med 2008;359:1555-1564.)

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