Influenza vaccination is an essential element of pregnancy care because pregnant women are at an increased risk of seasonal and pandemic flu. The effect of a mother's immunization against influenza during pregnancy is strongest when the infant is eight weeks old or younger, suggests new research.
However, the immunization effect decreased as the infants grew from eight to 16 weeks and 16 to 24 weeks, revealed the study.
‘The effect of a mother's immunization against influenza during pregnancy is strongest when the infant is eight weeks old or younger.’
In the study, the researchers followed infants born to women who participated in a randomized clinical trial of trivalent inactivated influenza vaccine (IIV3) when they were pregnant.
The study aimed to determine the vaccine's efficacy against influenza and infant antibody levels during their first six months of life. Study of the vaccine's efficacy included 1,026 infants born to women immunized with IIV3 and 1,023 infants born to women given placebo.
The vaccine's efficacy against influenza was highest at 85.6% when infants were eight weeks or younger, but decreased to 25.5% as the infants grew from eight to 16 weeks and to 30.3% among infants 16 to 24 weeks, according to the results.
"We and others have previously demonstrated that the administration of IIV3 during pregnancy confers protection against symptomatic influenza infection to the infants of the vaccinated mothers; here we show that the duration of this protection is likely to be limited to the first eight weeks of age," said Marta C. Nunes from University of the Witwatersrand in Johannesburg, South Africa.
Additionally, in a subset of infants, the percentage of infants with antibodies at or above a certain level dropped from 56% in the first week of life to less than 10% at 24 weeks of age.
The findings were published in the journal JAMA Pediatrics
The study suggests that the most likely mechanism of protection of the infants is through the transplacental transfer of maternal antibodies.