The Flu Shot During Pregnancy: Safe and Effective

A New Concern for Pregnant Women?

Earlier this fall, a Centers for Disease Control and Prevention (CDC)-funded study^[1] found a possible association between influenza vaccine and pregnancy loss. Specifically, this study found that pregnant women who received the influenza vaccine containing the pandemic H1N1 (H1N1pdm09) component and who also had been vaccinated the previous season with an H1N1pdm09-containing flu vaccine had an increased odds ratio of pregnancy loss within the first 28 days following vaccination.

Christopher M. Zahn, MD

Despite decades of research establishing influenza vaccination as a safe and essential part of prenatal care,^[2] the information presented in this study may cause some concern in pregnant women. However, it is important to highlight that even the CDC study authors acknowledge several significant limitations to the study. Furthermore, their own conclusion was that the study does not and cannot establish a causal relationship between repeated influenza vaccination and spontaneous abortion.^[1] That is why this flu season, women's healthcare providers play a particularly vital role in advising pregnant women to receive the influenza vaccination.

Pregnancy and Influenza

Changes in the immune system, heart, and lungs during pregnancy make pregnant women (and those up to 2 weeks postpartum) more prone to severe illness from influenza.^[3] Influenza during pregnancy increases the chance of hospitalization and adverse neonatal outcomes, such as low birthweight and preterm birth. Since 2010, the CDC estimates that flu has resulted in 140,000 to 710,000 hospitalizations annually.^[4] During the 2009 H1N1 influenza pandemic in the United States, pregnant women were more than four times more likely to be hospitalized than the general population and had higher mortality rates.^[5]

Vaccination during pregnancy is the best way to prevent serious illness caused by influenza. Flu vaccination during pregnancy also provides important protections to the fetus. Because the flu vaccine is not approved for use in infants younger than 6 months, maternal vaccination is the most effective strategy to protect infants. Through maternal vaccination, influenza antibodies are passed from the mother through the placenta to the fetus.^[6]

Safety Record of Influenza Vaccines

The safety of vaccination during pregnancy is of critical concern to obstetrician-gynecologists. The American College of Obstetricians and Gynecologists (ACOG) carefully tracks pregnancy-related vaccine safety information through its involvement in the National Vaccine Advisory Committee, the US Department of Health and Human Services, and the CDC's Advisory Committee on Immunizations Practices.

Millions of flu vaccines have been given for decades (including to pregnant women) with a strong safety record.^[7] For many years, ACOG and the CDC have recommended that every pregnant woman receive a flu shot, regardless of trimester of pregnancy. Multiple published studies, as well as clinical experience, support the belief that the flu vaccine is safe and effective during pregnancy.

The CDC study does not quantify the risk for miscarriage and does not prove that flu vaccine was the cause of miscarriage.^[8] Furthermore, the limitations of the study range from biased sampling to undocumented ultrasound dating. More research is needed to address the concern raised by this study.

Reassuring and Protecting Pregnant Women and Newborns

Despite these gaps and limitations, the CDC study received widespread attention in the media, and many women might now be confused about the flu vaccine or reluctant to receive it during pregnancy. It's essential that clinicians treating pregnant women are prepared to address these concerns and assure women that the flu vaccine is both safe and vital during pregnancy for the protection of both mother and baby. Pregnant women who forgo the flu vaccine risk serious illness for themselves and the fetus.

Continued research to improve knowledge around the flu vaccine's immediate and long-term effects is important to maintaining the safety and efficacy of the vaccine. Obstetric providers should continue to focus on established understanding about the risks associated with influenza during pregnancy and the safety and efficacy of the flu vaccination in protecting women and fetuses.

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