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When Is the Optimal Time to Vaccinate Against Flu in Pregnancy?

September 11, 2019

Women who receive the influenza vaccine later in pregnancy have stronger immune response and transfer more antibodies to the fetus compared with women who receive the vaccine earlier in pregnancy, according to a new analysis.

The analysis, which included women who were vaccinated with a seasonal or pandemic vaccine during different trimesters, evaluated the impact of antenatal vaccination timing on influenza vaccine immunogenicity during pregnancy, as well as on transplacental transfer to newborns.

To do this, the researchers collected data on geometric mean titer (GMT) measured by hemagglutination inhibition assays. For data to be included in the analysis, the trimester-stratified titers had to have been measured either at both pre‐vaccination and within 1 month post‐vaccination, at both post‐vaccination and delivery in the mother, or at delivery in cord blood or newborn blood.

In all, 16 studies from 4 online databases were included.

“Meta‐analyses found that, compared with women vaccinated in an earlier trimester, those vaccinated in a later trimester had a greater fold increase in hemagglutination inhibition [titers] (1.33‐ to 1.96‐fold) and higher hemagglutination inhibition [titers] in cord/newborn blood (1.21‐ to 1.64‐fold),” the authors wrote.

The researchers also determined that while a woman would be protected against the infection for a longer period of her pregnancy if she were vaccinated in an earlier stage of pregnancy, the immunogenicity may wear off by delivery. In turn, the researchers reported that their analysis’ findings support the current recommendation that these women receive a second dose if they are still pregnant during the following influenza season.

“We need to understand the full implications of vaccination timing for protection of mother, fetus, and newborn,” the researchers concluded. “A better understanding of the benefits of influenza vaccination during pregnancy may help increase vaccination rates among pregnant women.”

—Colleen Murphy


Cuningham W, Geard N, Fielding JE, et al. Optimal timing of influenza vaccine during pregnancy: A systematic review and meta‐analysis. Influenza Other Respir Viruses. 2019;13(5):438-452. doi:10.1111/irv.12649.

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