Mothers who get flu shots during pregnancy can provide protection to their newborns in their first weeks of life, according to a new report, but the protection might not last until the infants can be vaccinated themselves.
The study, conducted in South Africa, was an extension of an earlier study of the efficacy and safety in flu vaccinations during pregnancy.
“In 2014 we reported that immunization of pregnant women with trivalent inactivated influenza vaccine (IIV3) was safe, immunogenic, and partially protected the women with a vaccine efficacy of 50% and their infants with a vaccine efficacy of 49% against laboratory-confirmed influenza illness during a 6-month follow-up postdelivery,” says study author Marta C Nunes, PhD, of the University of Witwatersrand in Johannesburg, South Africa. “The exact duration of the protection in the infants was, however, never assessed and is generally described during the overall follow-up period (often the first 6 months of life).”
The new report, however, shows that vaccine efficacy is highest in the first 8 weeks of an infant’s life, and most likely protects them after birth by transplacental transfer of maternal antibodies or through breast milk.
The study evaluated infants born to women who had participated in a randomized, double-blind, placebo-controlled clinical trial between 2011 and 2012 that aimed to test the efficacy of IIV3 during pregnancy. Although the initial goal of the study was to determine how well the flu shot worked for pregnant women, researchers also performed a secondary analysis measuring hemagglutination inhibition (HAI) antibodies.
Researchers studied 1026 infants born to IIV3 recipients and 1023 infants born to mothers given placebo from the efficacy study. In the group of infants born to vaccinated mothers, efficacy against polymerase chain reaction (PCR)-confirmed influenza was 85.6% in infants aged 8 weeks or younger; 25.5% among infants aged 8 to 16 weeks; and 30.3% among infants aged 16 to 24 weeks. Immunity against influenza was detected via HAI titers as well, with 56% of infants in the IIV3 test group showing titers in the first week of life compared with less than 40% by 16 weeks and less than 10% by 24 weeks of age.
Influenza surveillance was performed using HAI assays performed at ages 7 days, 8 weeks, 16 weeks, and 24 weeks, as well as weekly surveillance for respiratory illness.
The study also reviewed what types of antibody concentrations were highest, with HAI titers at 78.3% for the A/H1N1pdm09 strain; 56.6% for the A/H3N2 strain; and 81.1%
for the B/Victoria strain among infants whose mothers were vaccinated during pregnancy. In the placebo group, HAI titers were 33.6% for the A/H1N1pdm09 strain; 17.3% for the A/H3N2 strain; and 41.8% for the B/Victoria strain.
Titers decreased, however, as the infants aged, both in the vaccinated and placebo groups. By 16 weeks, HAI titers in the group born to vaccinated mothers dropped to 39.5%, 19.1%, and 40% for the 3 strains (A/H1N1pdm09 strain, the A/H3N2 strain, and the B/Victoria strain, respectively), and to 10%, 6.7%, and 9.4% by 24 weeks. In the placebo group the decline was similar, with 5% to 9% at 16 weeks and 3.5% at 24 weeks.