Infants under the age of 6 months cannot receive flu vaccination, so protection against the virus is established through immunization their mothers receive during pregnancy. But exactly how long does maternal flu vaccination protect infants after birth? According to new research published in JAMA Pediatrics, infants may only receive adequate protection against the virus in the first 8 weeks of life.

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Researchers say maternal flu vaccination may only protect infants for 8 weeks after birth.

The Centers for Disease Control and Prevention (CDC) state that, among children of all ages, those under 6 months of age are at the greatest risk of being hospitalized from flu.

Because flu vaccination is only approved from the age of 6 months, it is recommended that expectant mothers receive a flu shot in order to protect themselves and their newborn children from flu-related complications.

But according to study co-author Marta C. Nunes, Ph.D., of the University of the Witwatersrand in Johannesburg, South Africa, and colleagues, exactly how long maternal flu vaccination protects an infant after birth has been unclear.

With a view to finding out, the team analyzed data of more than 2,000 infants born to mothers who took part in a randomized clinical trial assessing the efficacy of maternal flu vaccination.

A total of 1,026 infants were born to mothers who received the trivalent inactivated influenza vaccine (IIV3) during pregnancy, while 1,023 infants were born to mothers who received a placebo.

As part of the trial, blood samples of the infants were taken 7 days, 8 weeks, 16 weeks, and 24 weeks after birth. These were assessed for the presence of hemagglutination inhibition (HAI) antibodies - an indicator of protection against flu, with high levels signaling higher protection.

The infants were also regularly assessed for physical symptoms of flu up to the age of 6 months.

Flu protection fell to 25 percent 8-16 weeks after birth

As demonstrated in previous studies, the researchers found that maternal flu vaccination offered infants some level of protection against the virus up to the age of 6 months, but they found the level of protection reduced dramatically after the first 2 months.

Maternal flu vaccination was found to be 85.6 percent effective in protecting infants against the virus in the first 8 weeks of life. However, efficacy fell to just 25.5 percent from 8-16 weeks of age, increasing slightly to 30.3 percent at 16-24 weeks of age.

What is more, among infants born to mothers who received the flu vaccine in pregnancy, the team identified a significant drop in HAI levels the first week of life, from 56 percent to just 10 percent at the age of 24 weeks.

Based on their results, the researchers say flu vaccination in pregnancy may not offer effective protection for infants after the first 8 weeks of life:

"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."

"Several potential mechanisms of protection have been proposed. [...] Our study suggests that the most likely mechanism of protection of the infants is through the transplacental transfer of maternal antibodies," they add.

The team says the results highlight the need to identify alternative strategies that protect infants against flu from 8 weeks of age.

In an editorial linked to the study, Dr. Flor Munoz, of the Baylor College of Medicine in Houston, TX, says the study from Nunes and colleagues "contributes significantly to our understanding of infant protection against influenza through maternal vaccination," noting that it provides more "conclusive information" that the maternal flu vaccination may offer limited efficacy to young infants.

"[...] until safe and effective influenza vaccines can be given to infants younger than 6 months of age, it is imperative that passive protection is optimized," she adds.

Read about a study that found 97 percent of flu-infected infants are born to non-vaccinated mothers.