Immune Response of Pregnant Women to Influenza Vaccination Declines as Pregnancy Progresses

NIAID Now | January 25, 2019

The seasonal influenza (flu) vaccine is recommended for all pregnant women to protect them and their newborn infants from illness. It also reduces the risk of pregnancy complications such as premature birth and low birthweight that can result in short- and long-term health problems for the child.

Influenza is more likely to cause severe illness or death in pregnant women than in women of the same age who are not pregnant, especially during the later stages of pregnancy, suggesting that pregnancy dampens the immune response to the flu virus. Researchers therefore wondered whether a woman’s immune response to seasonal influenza vaccination also declines during pregnancy, potentially reducing the vaccine’s effectiveness.

To help answer this question, a research team funded in part by NIAID studied the antibody response to influenza vaccination throughout pregnancy. The researchers collected blood samples before and after vaccination from 71 healthy pregnant women ages 18–39 and 67 healthy women of the same age range who were not pregnant. They evaluated the blood levels of several types of anti-hemagglutinin (HA) antibodies, which are thought to provide protective immunity against the virus by targeting the HA protein on its surface. At the time of vaccination, 13 of the 71 pregnant women in the study were in their first trimester of pregnancy, 43 were in the second trimester, and 15 were in the third trimester. Most study participants had been previously vaccinated against influenza.

Overall, the team found that anti-HA antibody levels were lower in pregnant women compared to non-pregnant women and these antibody responses decreased as women were vaccinated later in pregnancy. The findings suggest that pregnant women produce a weaker antibody response to some forms of the HA protein when vaccinated in the later stages of pregnancy, correlating with the increased rate of influenza hospitalizations and complications seen as delivery approaches. According to the researchers, these results suggest a need to reconsider the timing of administering the influenza vaccine to pregnant women. Earlier vaccination may lead to better antibody responses to the vaccine and improved protection against influenza.

Reference: Schlaudecker E et al. Declining responsiveness to influenza vaccination with progression of human pregnancy. Vaccine. 2018 Jul 25;36(31):4734-4741.

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