HONOLULU – When women are given influenza vaccine in their third trimester of pregnancy, their infants receive protection against flu infection, results of a randomized controlled trial of more than 300 pregnant women confirm.
“This is the first randomized controlled trial of maternal immunization with influenza vaccine,” Dr. Mark C. Steinhoff reported at the annual meeting of the Pediatric Academic Societies. “Although [maternal immunization] is a U.S. government policy, it's one of the few not based on randomized controlled trials.”
The study was part of the Maternal Gift Study, which involved 340 pregnant women and 331 live births in a middle-class urban population in Bangladesh. Women in the study were randomized to receive either influenza vaccine or pneumococcal conjugate vaccine during their third trimester of pregnancy. For the purposes of this analysis, the investigators used the mother-infant pairs receiving pneumococcal vaccine as the control group.
The mothers were an average 25 years old, and were vaccinated an average 55 days before giving birth. Ninety-two percent gave birth in a hospital or clinic, 46% by cesarean delivery. The infants averaged just above 3 kg at birth and were breast-fed exclusively an average of 14 weeks.
The investigators looked both at proven influenza illness and at all febrile respiratory illnesses as outcome measures. The trivalent influenza vaccine was associated with a 63% reduction in proven influenza in infants 0-6 months of age and a 30% reduction in all febrile respiratory illnesses in infants and their mothers.
The fact that the influenza vaccine was compared with the pneumococcal vaccine and not with placebo probably resulted in an underestimate of the influenza vaccine's effectiveness, said Dr. Steinhoff of Johns Hopkins University, Baltimore. “It's possible that pneumococcal vaccine could reduce some of the viral illnesses.”
Furthermore, the vaccine's protective effect appeared to last at least until the infants were 5 months old. This is particularly important because current U.S. guidelines do not recommend influenza vaccine for children younger than 6 months old.
And it's those very children who are responsible for almost half of childhood influenza hospitalizations. According to one study, children 0-6 months old accounted for 48% of all the influenza hospitalizations among children below the age of 5 years (N. Engl. J. Med. 2006;355:31-40).
Dr. Steinhoff disclosed that he has served on Sanofi's speakers' bureau and has received research support from Sanofi-Aventis, Wyeth, and Merck & Co. “None of these interactions had any bearing on this particular study.”