Johannesburg, Jul 6 (IANS): The effect of a mother's immunisation against influenza during pregnancy is strongest when the infant is eight weeks old or younger, suggests new research.

However, the immunisation effect decreased as the infants grew from eight to 16 weeks and 16 to 24 weeks, revealed the study.

In the study, the researchers followed infants born to women who participated in a randomised 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 per cent when infants were eight weeks or younger, but decreased to 25.5 per cent as the infants grew from eight to 16 weeks and to 30.3 per cent 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 8 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 per cent in the first week of life to less than 10 per cent 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.