Flu Vaccination & 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

Influenza (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, including illness resulting in hospitalization. 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.

Numerous studies have shown that flu vaccination protects pregnant women during and after pregnancy and also protects babies from flu infection for several months after birth, before the child is old enough to be vaccinated (mom passes antibodies onto the developing baby during her pregnancy).

Because pregnant women are at high risk of serious flu complications, CDC recommends influenza vaccination during any trimester of pregnancy. In the United States, the influenza season typically occurs from October to May. The CDC’s Advisory Committee on Immunization Practices (ACIP) recommends that all adults receive an annual influenza vaccine; both ACIP and the American College of Obstetricians and Gynecologists recommend 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. Numerous studies, including clinical trials and observational studies, and data from vaccine safety monitoring systems have demonstrated consistently the safety of influenza vaccination during pregnancy.

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

On February 27, 2019, researchers presented findings to the ACIP from a CDC-funded study in the Vaccine Safety Datalink (VSD) which analyzed data from three influenza seasons (2012-13, 2013-14, 2014-15) looking for any association among pregnant women who had received a flu vaccine during their pregnancy and miscarriage, for which the medical term is spontaneous abortion. The study found NO increased risk for spontaneous abortion after influenza vaccination during pregnancy.

This study was conducted in follow-up to a previous VSD study that was presented publicly at an ACIP meeting in June 2015 and published in a peer-reviewed journal in 2017. That study had identified an association between flu vaccination early in pregnancy and spontaneous abortion. The association was only observed in women who had been vaccinated in the previous influenza season with a pH1N1-containing vaccine. The study had several notable limitations, including a small sample size that could have led to imprecise results. Nonetheless, the June 2015 study was the first analysis to show that association; earlier studies had not shown the association. In response to this association (referred to as a “safety signal”), CDC provided funding for a follow-up Vaccine Safety Datalink (VSD) study that included about three times as many women as the June 2015 study.

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 approximately 12 million persons per year. The health information includes demographic characteristics, vaccinations, and medical outcomes.

The study that was presented to the ACIP in February 2019 will be submitted for publication in a peer-reviewed journal. Slides presented to the ACIP are available on the ACIP web site.

CDC Recommendation

CDC and ACIP 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 presented in February 2019

  • 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).
  • The frequency of prior miscarriage was similar for cases and controls.
  • This study used similar methods to the study presented in June 2015, but analyzed data from more recent influenza seasons, was considerably larger, and was designed to assess prior season vaccination.
  • Main finding: There was no association between and influenza vaccination and spontaneous abortion.

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 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 hospitalizationexternal icon when infection does occur and is the primary preventive intervention for 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, 2017external icon) 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.