Addressing Concerns Pregnant Women Might Have about Flu Vaccine Safety

Information & Guidance for Health Care Providers

Health care providers of pregnant women play a vital role in advising their pregnant patients on how to protect themselves and their developing babies against many threats, including influenza (flu). This fact sheet contains information about influenza and influenza vaccination during pregnancy and provides guidance on how to address concerns that patients may have about influenza vaccination.

Background

Flu is more likely to cause severe illness in pregnant women than in women who are not pregnant, particularly during the second and third trimesters. Changes in the immune system, heart, and lungs during pregnancy make pregnant women (and women up to two weeks postpartum) more prone to severe illness from flu. Flu also may be harmful for a pregnant woman’s developing baby. A common flu symptom is fever, which may be associated with neural tube defects and other adverse outcomes for a developing baby.

A number of studies have shown that flu vaccination can protect pregnant women during and after pregnancy, and also protect her baby from flu infection for several months after birth, before he or she is old enough to be vaccinated (mom passes antibodies to the developing baby during her pregnancy).

Because pregnant women are at high risk of serious flu complications, they are recommended for influenza vaccination during any trimester of their pregnancy In the United States, the influenza season typically occurs from October to May. The CDC’s Advisory Committee on Immunization Practices and the American College of Obstetricians and Gynecologists recommend that all adults receive an annual influenza vaccine and that women who are or will be pregnant during influenza season receive an inactivated influenza vaccine as soon as it is available. Millions of flu vaccines have been given for decades, including to pregnant women, with a strong safety record. Numerous studies, including clinical trials and observational studies, and data from safety reporting systems have demonstrated consistently the safety of influenza vaccination during pregnancy.

Getting a Flu Vaccine While Pregnant Does NOT Increase the Risk of Miscarriage

One of the largest and strongest studies examining influenza vaccination and risk of miscarriage was conducted in CDC’s Vaccine Safety Datalink (VSD) project. The studyexternal icon covered three influenza seasons (2012-13, 2013-14, 2014-15) looking for any increased risk for miscarriage among pregnant women who had received a flu vaccine during their pregnancy. The study found NO increased risk for miscarriage after influenza vaccination during pregnancy. These results support the current ACIP policy that recommends influenza vaccination at any time during pregnancy.

This study was conducted in follow-up to a previous smaller study. The prior studyexternal icon had suggested the possibility that women who received H1N1-containing flu vaccine two years in a row had an increased risk of miscarriage.

VSD is a collaboration between CDC’s Immunization Safety Office (ISO) and several integrated healthcare organizations across the United States. It uses electronic health information from more than 9 million people, approximately 3% of the US population. The health information includes demographic characteristics, vaccinations, and medical outcomes.

See Study Details for more information about these studies.

CDC Recommendation

CDC and ACIP continue to recommend that pregnant women get a flu vaccine during any trimester of their pregnancy because flu poses a danger to pregnant women and a flu vaccine can prevent influenza in pregnant women.

CDC Guidance

As always, health care decisions should be part of an ongoing discussion between provider and patient. CDC recommends that any pregnant woman who has questions about vaccines talk to her health care provider.

Study Details

Study published in 2019

Article Title: Inactivated influenza vaccine and spontaneous abortion in the Vaccine Safety Datalink in 2012–13, 2013–14, and 2014–15.external icon

  • The study included pregnant women at participating VSD sites in Washington, California, Colorado, Oregon, and Wisconsin, and spanned three influenza seasons: 2012-13, 2013-14, 2014-15.
  • Eligible women were 18-44 years old with at least 20 months of enrollment in the health plan.
  • This was an observational study using existing medical records and immunization data. Women were not contacted.
  • This was a matched case-control study. Women with miscarriage from 6-19 weeks gestational age (cases) were individually matched to other women who had a live birth (controls). The case-control pairs were matched on age group, VSD site, date of last menstrual period, and influenza vaccination status during the preceding season. The analysis compared the influenza vaccination status during pregnancy of cases and controls after adjusting for other factors that may be associated with vaccination status and miscarriage.
  • Medical records were reviewed to confirm miscarriage, estimate date of fetal loss, and determine dates of vaccine receipt.
  • The primary analysis included 1,236 eligible matched pairs, including 627 pairs who were vaccinated in the previous season and 609 pairs who were not.
  • The median gestational age at the time of miscarriage was 7 weeks (same as prior study).
  • Women with miscarriage were significantly older and had higher body mass index than controls. There were also significant differences in race/ethnicity. The frequency of prior miscarriage was similar for cases and controls.
  • Main finding: There was no significant association between miscarriage and influenza vaccination.
  • The limitations of this study include:
    • A challenge common to all studies of miscarriage is estimation of the miscarriage date. The researchers attempted to estimate the date of pregnancy loss when possible and relied on an algorithm developed and refined in the previous two studies, as well as guidance from an obstetrician to integrate various types of information from the medical record, such as ultrasound results, clinical and laboratory findings, and provider notes. Researchers estimated miscarriage dates without knowledge of vaccination status, so any misclassification should be unrelated to exposure status.
    • Misclassification of vaccination status is possible, particularly for women who appeared to be unvaccinated, since influenza vaccination may be available outside of healthcare systems. However, given the strong recommendations for vaccination of pregnant women, we expect that out-of-system vaccinations would be documented by the provider in the medical record.
    • Women in the study were members of their respective healthcare organization for at least 20 months prior to their last menstrual period, and may not be representative of all pregnant women.

Study published in 2017

Article Title: Association of Spontaneous Abortion with Receipt of Inactivated Influenza Vaccine Containing H1N1pdm09 in 2010-11 and 2011-12external icon.

This study examined data from the 2010-2011 and 2011-2012 influenza seasons and identified an association between flu vaccination early in pregnancy and an increased risk of spontaneous abortion or miscarriage; particularly among women who had received influenza vaccine during the previous influenza season. However, the study had several limitations, including small sample size which could have led to imprecise results. This study was the only analysis to show that association; no other studies had found an increased risk of SAB following influenza vaccination. In response to this association (referred to as a “safety signal”), CDC provided funding for the larger 2019 follow-up Vaccine Safety Datalink (VSD) study that included about three times as many women.

A Flu Vaccine is the Best Protection Against Flu

Getting a flu vaccine is the first and most important step in protecting against flu. Pregnant women should get a flu shot. It is recommended that pregnant women receive the inactivated influenza vaccination. Flu vaccines given during pregnancy protect both the mother and her baby from flu. Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant women by about 50 percent. Pregnant women who get a flu vaccine are helping to protect their babies from flu illness for the first several months after their birth, when they are too young to get vaccinated. There are a number of recent studies that show the benefits of flu vaccination for pregnant women. Influenza vaccination can reduce the risk of influenza-associated hospitalization and is the primary way to prevent flu in pregnant women.

Earlier Studies That Support the Safety of Flu Vaccination in Pregnant Women
  1. Reviews of reports to the Vaccine Adverse Reporting System (VAERS) (Moro et al, 2011external icon and Moro et al, 2017) found no unusual or unexpected patterns of reporting for pregnancy complications or adverse fetal outcomes among pregnant women and flu shots.
  2. A study using VSD data (Irving et al, 2013external icon) found no increased risk of miscarriage among pregnant women who received flu vaccines in the 2005-06 or 2006-07 flu seasons.
  3. A large study using VSD data (Kharbanda et al, 2013external icon) found no increased risk for adverse obstetric events (like chorioamnionitis, pre-eclampsia, or gestational hypertension) for pregnant women who received the flu vaccine from 2002 to 2009 when compared to pregnant woman who were not vaccinated.
  4. A VSD study (Nordin et al, 2014external icon) compared pregnant women who received the flu shot with an equal number of pregnant women who did not receive the flu shot during the 2004-05 and 2008-09 flu seasons. The study found no differences between the two groups in the rates of premature delivery or small for gestational age infants.
  5. A large August 2017 study using VSD dataexternal icon found that the babies of women who received the flu shot during their first trimester had no increased risk of having children with major birth defects.