Vaccinating pregnant women against seasonal influenza reduced the risk of laboratory-confirmed influenza infection in their infants by 41%, according to a study published online Oct. 4 in the Archives of Pediatric and Adolescent Medicine.
Maternal immunization similarly cut by 39% the risk that infants up to 6 months of age would be hospitalized for influenza-like illness, said Angelia A. Eick, Ph.D., of the Center for American Indian Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, and her associates.
Influenza vaccination is already recommended for pregnant women to reduce their risk of developing flu-related complications. “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,” the investigators noted.
Even though such immunization is recommended during pregnancy, it is not well accepted in the United States and many pregnant women do not get vaccinated. Since it would be unethical to perform a randomized, controlled study of maternal vaccination, Dr. Eick and her colleagues conducted a nonrandomized observational study to assess whether immunization during pregnancy conferred protection to infants.
The study subjects were 1,160 mother-infant pairs in which approximately half the mothers (573) had chosen to receive seasonal flu vaccine while pregnant and the other half (587) had declined the vaccine. All were enrolled after delivering healthy singleton infants at 7 hospitals serving the Navajo and White Mountain Apache Indian reservations in the southwestern United States during three flu seasons between 2002 and 2005.
A total of 605 infants developed influenza-like illness during the flu season following delivery. “We found a 41% reduction in the risk of laboratory-confirmed influenza virus infection for infants of influenza-vaccinated mothers compared with infants of unvaccinated mothers,” the researchers said (Arch Pediatr. Adolesc. Med. 2010 [doi10.1001/archpediatrics.2010.192]).
The incidence of influenza-like illness was 6.7 per 1,000 person-days for infants of mothers who had been vaccinated, compared with 7.2 per 1,000 person-days for infants of mothers who had not.
Among the infants whose mothers were vaccinated, there was a 41% reduction in the risk of laboratory-confirmed influenza virus infection compared with those whose mothers declined vaccination.
When the analysis was restricted only to cases of influenza that required hospitalization, a 39% reduction in risk was found for infants of women who had been vaccinated, compared with those of mothers who had not been vaccinated.
Cord blood samples or infant blood samples taken at 2-3 months of age were available for 160 study subjects. In this subgroup, the risk of influenza infection declined with increasing antibody titers.
The exact mechanism by which vaccination of the mother conferred protection to the infant is not certain. It may be due to maternal influenza antibodies being acquired transplacentally or through breastfeeding, or to reduced infant exposure to influenza in the mother. It even could be due to residual confounding not accounted for in the statistical analyses, but the finding of significantly higher antibody titers in 2- to 3-month-old infants who did not develop illness argues against that possibility, Dr. Eick and her colleagues said.
Disclosures: The study was funded by the U.S. Department of Health and Human Services’ National Vaccine Program Office, the Office of Minority and Women’s Health (now the Office of Health Disparities), the Centers for Disease Control and Prevention, Aventis-Pasteur, and Evans-Powderject. One of Dr. Eick’s associates reported ties with MedImmune, Pfizer, and Sanofi-Pasteur, all of which manufacture influenza vaccine.