Evaluating the Safety of Flu Vaccination in Pregnancy
Sep 18, 2017

The decision to get a flu vaccination in pregnancy is one that should be based on a complete evaluation of the scientific evidence that is available.  Flu shots have been safely administered to millions of pregnant women over many years, so how should expectant parents respond to a recent study that implies a connection between multiple flu vaccinations and the incidence of miscarriage in early pregnancy?  

To properly evaluate the significance of the latest data, we must consider the findings of this one report alongside the abundance of other science-based information we have, such as: 

1. Why the Advisory Committee on Immunization Practices (ACIP) currently recommends flu vaccination among pregnant women.
2. Data from the numerous studies that support the safety of the ACIP’s current recommendation of flu vaccine for pregnant women.
3. Details of the “case-control” study in question and an examination of the study methods, findings and limitations.

Why the ACIP recommends flu vaccination among pregnant women.  

Currently the CDC’s Advisory Committee on Immunization Practices (ACIP) recommends that pregnant women get a flu vaccine during any trimester of pregnancy to help protect them and their newborns from the dangers of influenza.  Due to changes in the immune system, heart and lungs during pregnancy, expectant women are more prone to severe illness from flu, which has been known to result in premature delivery, low birth weight babies, miscarriage, hospitalization or even death.

Flu vaccination in pregnancy doesn’t just help protect the expectant mother from influenza, it is also the most effective way to pass critical immunity on to the baby during pregnancy.  This passive immunity can then protect the infant child from the dangers of influenza in the time before they are old enough to receive their own flu vaccination at six months of age.

The ACIP recommendation for flu vaccination during pregnancy is supported by other organizations as well, to include The American College of Obstetricians and Gynecologists (ACOG) and the American College of Nurse-Midwives (ACNM).

The studies that support the safety of flu vaccination in pregnancy. 

The ACIP is a committee which consists of 15 voting members who have expertise in vaccinology, immunology, pediatrics, internal medicine, nursing, family medicine, virology, public health, infectious diseases, and preventive medicine.  The Committee meets in person three times a year and subcommittees meet regularly throughout the year via conference call to discuss vaccine research and scientific data related to vaccine effectiveness and safety.

The current ACIP recommendation for flu vaccination during pregnancy is based on a thorough review of the evidence compiled from numerous studies, which include the following:

• • A review of reports to the Vaccine Adverse Reporting System (VAERS), a national vaccine safety surveillance program run by CDC and the Food and Drug Administration (FDA), (Moro et al, 2011) which found no unusual or unexpected patterns of reporting for pregnancy complications or adverse fetal outcomes among pregnant women and flu shots.

• • A study using Vaccine Safety Datalink (VSD) data (Irving et al, 2013) which found no increased risk of miscarriage among pregnant women who received flu vaccines in the 2005-06 or 2006-07 flu seasons. (The VSD is a collaborative program that monitors the safety of vaccines and conducts studies about rare and serious adverse events following immunization.)

• • A large study using VSD data (Kharbanda et al, 2013) which 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 compared to pregnant woman who were not vaccinated.

• • A VSD study (Nordin et al, 2014) which 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.

• • A large August 2017 study using VSD data which 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.

The examination of vaccine safety is an ongoing process.  Before being approved for administration, vaccines undergo rigorous testing by their manufacturers, the FDA, and the FDA’s Center for Biologics Evaluation and Research. Clinical trials are performed before the vaccine is made available to the public, to confirm the vaccine’s safety and efficacy. Even after the vaccine receives FDA-approval, post-licensure studies are conducted on an ongoing basis to continually monitor the vaccine’s safety and to detect and respond to any rare adverse events.

While the studies conducted to date have not signaled any safety concerns, the ACIP and the CDC are committed to the continuous evaluation of the safety of all vaccines, to include those recommended for pregnant women.

This has led to the “case-control” study of flu vaccination and possible miscarriage which was recently published in the journal Vaccine on September 12, 2017 and reported on by The Washington Post, the Associated Press, and various other media outlets.  The study showed that women in early pregnancy who received two consecutive annual vaccines during 2010-11 and 2011-12, both of which included a 2009 pandemic H1N1 (H1N1pdm09) component, had an increased risk of spontaneous abortion (miscarriage) in the 28 days after receiving the second vaccine.

Details of the recently published study of women who had miscarriage following flu vaccination. 

The study, which is now being discussed as a “safety signal”, used vaccine safety data collected through the VSD to compare 485 women aged 18-44 who miscarried to 485 pregnant women aged 18-44 who did not miscarry.

Although the study was only published this week, the preliminary results had already been discussed at a public meeting of the ACIP two years ago.  At that time, the Committee reviewed and discussed the study limitations and determined that while the study results warranted further investigation, this one study was not statistically significant enough to prompt a change in the vaccine recommendation.

As the study has begun circulating on social media, and expectant parents have expressed concerns, the CDC, along with many medical experts, have highlighted the studies’ limitations by noting the following:

• • The study examined data from a small number of women in a subgroup who received H1N1-containing vaccines in consecutive years. The small numbers in the study could have led to imprecise results.

• • It is possible that women who have an increased risk for miscarriage might also be more likely to have received influenza vaccine. These conditions could have made the women more likely to miscarry.

• • Many miscarriages occur early in pregnancy and may not come to medical attention. The impact on the study findings of miscarriages that were not identified is unknown.

• • Many women in the study had risk factors for miscarriage (smokers, etc.)
• • It is not known how many women in the study were aware that they were pregnant at the time of vaccination and therefore could have been participating in risky behaviors that could have contributed to miscarriage.

• • Outside of this study there is a traditionally high miscarriage rate for early pregnancies, regardless of vaccination and other variables.

• • Flu vaccinations could have been incompletely recorded because some women could have received flu shots in another setting. The possible impact of unidentified vaccinations is unknown. However, this effect cannot account for the observed association if unidentified vaccinations occurred with similar frequency in cases and controls.

• • This study was a case-control study that estimated an odds ratio of vaccination among women who had a miscarriage compared to those who did not. The study did not estimate risk of miscarriage after influenza vaccination. Therefore, the findings cannot be used to estimate the probability of miscarriages for pregnant women who received H1N1-containing flu vaccinations two years in a row.

• • The study does not quantify the risk of miscarriage and does not prove that flu vaccine was the cause of the miscarriage.

Current recommendations: 

While it’s understandable that this recent study may give people a reason to pause, it’s important to understand the scientific process.  We must evaluate the latest published data in the context of an abundance of other data as well. As new information becomes available and additional studies are conducted to further investigate possible safety concerns, the ACIP will continue to examine their recommendations.

As always, pregnant women should speak with their healthcare providers about any questions they may have regarding their healthcare.

In the meantime, expectant parents may be interested in reading more about this topic on the following resources: 

• Study Abstract
• ACOG’s Immunization For Women website
• ACOG Statement
• CDC Pregnant Women & Influenza
• CDC: Flu Vaccination & Possibly Safety Signal
• ACIP Meeting Information (Agenda, Minutes, Presentations Slides) 
• The Washington Post article
• Associated Press article 
• Daily Beast article by Dr. Paul Offit

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