Pregnancy Heightens Immune Response to Influenza, Making Vaccination a Key Preventive Measure


Pregnant women exhibit a more powerful immune response to influenza, but only half are currently vaccinated.

Pregnant women have an unexpectedly high immune response to influenza, which may contribute to more severe flu symptoms, according to researchers from Stanford University and Lucile Packard Children’s Hospital.1

For their study published September 22, 2014, in the Proceedings of the National Academy of Sciences, the researchers compared the immune cells of 21 nonpregnant women to those of 29 pregnant women. The pregnant women were not only more responsive to influenza viruses immunologically, but also had a higher level of immune response 7 days after receiving the influenza vaccine than their nonpregnant counterparts.1

The study suggested that the natural killer and T cells in pregnant women have an enhanced response to the H1N1 and H3N2 influenza strains compared with the same cells in nonpregnant women.1 Clinically, a higher level of immune response to influenza indicates a more severe infection.

“We now understand that severe influenza in pregnancy is a hyperinflammatory disease, rather than a state of immunodeficiency,” said senior author Catherine Blish, MD, PhD, in a press release on the findings. “This means that treatment of flu in pregnancy might have more to do with modulating the immune response than worrying about viral replication.”

At the same time, however, pregnant women who receive the influenza vaccine are more likely to develop protective antibodies against the viruses. Given the infection is associated with poor pregnancy outcomes—including a quadrupled risk for premature delivery—the researchers hope to bring attention to the importance of annual influenza vaccination, particularly among pregnant women.1 Of concern, “only 50% of pregnant women are currently vaccinated for influenza,” said lead study author Alexander Kay, MD.

An additional benefit of receiving the influenza vaccine during pregnancy is that it may help the fetus develop passive immunity to the viruses, protecting a neonate from influenza during the 6 months of life when it cannot receive the vaccine.2

The US Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices and the American College of Obstetricians and Gynecologists recommend that all pregnant women without contraindications receive an influenza vaccine annually. According to a recent statement from a workgroup of the joint organizational panel, women should be vaccinated “as soon as it is available.”3

Underestimating the threat of influenza is a common attitude among pregnant women. According to the results of a recent systematic review, pregnant women are more likely to receive the recommended vaccine if they understand their increased risk for influenza, as well as the fact that infection may lead to pregnancy complications. Consequently, all pregnant women should be educated about the importance of influenza vaccination to improve their health and curb the viruses’ effect on pregnancy outcomes.4


1. Kay AW, Fukuyama J, Aziz N, Dekker CL, Mackey S, Swan GE, Davis MM, Holmes S, Blish CA. Enhanced natural killer-cell and T-cell responses to influenza A virus during pregnancy. PNAS. 2014 Sept 22.

2. Kalenik B, Sawicka R, Góra-Sochacka A, Sirko A. Influenza prevention and treatment by passive immunization. Acta Biochim Pol. 2014.

3. CDC/ACOG. Committee opinion no. 608: influenza vaccination during pregnancy. Obstet Gynecol. 2014;124(3):648-651.

4. Yuen CY, Tarrant M. Determinants of uptake of influenza vaccination among pregnant women - a systematic review. Vaccine. 2014;32(36):4602-4613.

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