How mom's flu shot protects baby

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.

In a study published in JAMA Pediatrics, researchers studied the effects of influenza vaccines given to mothers during pregnancy, and evaluated how long protections lasted for infants after birth.

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.

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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.

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In terms of actual illness, 37 infants born to mothers in the placebo group and 19 infants born to mothers in the IIV3 group experienced PCR-confirmed influenza illness during the study period.

The infants involved in the study were born an average of 81 days after their mothers were vaccinated with IIV3, and were monitored for influenza infection for about 172 days after birth.

“We have previously shown that a higher transplacental antibody transfer was achieved with a longer interval between vaccination and delivery, which suggests that a prolonged cumulative transfer of antibodies leads to higher quantity of antibodies at birth compared with a shorter period during maximal transplacental transfer,” the researchers note. “Our study finds that to achieve better infant protection, vaccination during pregnancy should not be delayed until the third trimester.”

The timing of maternal vaccination is important, according to the study, particularly because there is no vaccination option for children aged younger than 6 months. Therefore, enhancing the concentration of antibodies passed to the infant and finding more immunogenic vaccines that can generate protective responses in infants is key, the researchers say.

“Alternatively, more immunogenic vaccines that can generate a protective immune response in infants, beginning at 8 weeks of age, need to be identified,” the authors note. “Importantly, whereas protection conferred by vaccination is likely to be mediated by both antibodies and cellular-immune responses in vaccine recipients, passive protection of infants is almost exclusively mediated by antibodies.”

Efforts to prevent influenza infections are important in this population because there is a high incidence of influenza infection in infants aged younger than 6 months. Influenza vaccines are only approved for use in infants aged 6 months and older, leaving younger infants at risk of developing dangerous infections. Because of their immature immune systems, infants are at increased risk for influenza infection and hospitalization associated with influenza infection.

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Since vaccination is not an option, studying maternal vaccination is the next step. A similar recommendation recently published in Pediatrics reveals that similar techniques are used to fight pertussis. Researchers say that maternal vaccination early in the third trimester-around 28 weeks-is ideal.

In the influenza study, researchers say maternal vaccination is an effective and safe way to prevent what could be a devastating disease for newborns who can’t be protected by a vaccine.

“Immunization of pregnant women with trivalent inactivated influenza vaccine is safe, immunogenic, and efficacious in protecting the women and their infants against influenza illness. Even if this protection is shorter than previously estimated, very high protection is achieved for the first 8 weeks of life,” Nunes says.