May 25, 2018
Doctor vaccinating young pregnant woman
Health News

Does the Flu Shot Lead to Miscarriage?

by Amanda Z. Naprawa  

Recent headlines about a purported link between the flu vaccine and miscarriages are sure to scare many pregnant women—possibly into avoiding the vaccine this year. Here’s why those headlines are entirely misleading, and what all pregnant women need to know instead.

The attention-grabbing headlines come from a study published Sept. 25 in the journal Vaccine. Researchers examined data from almost 1,000 women enrolled in one of six health care plans, who were identified by medical records as having been pregnant during the 2010-2011 or 2011-2012 influenza seasons. Half of the women had a confirmed miscarriage between 5 and 20 weeks of pregnancy. The other half (the “controls”) either gave birth or had a stillbirth (that is, they did not miscarry). The groups were matched on age, geographic location, and date of last known menstrual period.

The researchers found that women who had experienced a miscarriage were more likely to have been vaccinated against the flu in the preceding 28 days compared with women who did not have a miscarriage. Interestingly, this pattern was only found among women who had also been vaccinated against flu the previous year.

Setting the record straight

While the results of the study may warrant further investigation, they do not warrant changing the current recommendations of public health and medical organizations that strongly encourage pregnant women to get the flu vaccine. For one thing, the study’s methodology had flaws. The researchers used what’s called a “case-control” design, in which one group (cases—in this instance women who’d had a miscarriage) is compared with another group (controls—in this instance women who were pregnant and did not have a miscarriage). In any case-control study, it is important that the cases and controls match on all other variables (age, weight, health, etc) to make sure that there are no other reasons that could explain the difference between the two groups (in this case, the likelilihood of having had the flu shot in the last 28 days).

In the current study, however, the cases differed from the controls on several important variables: They were much more likely to be older, to have had a history of smoking, to be African American, and to have had a history of more than two miscarriages—all of which are independent risk factors for miscarriage.

Additionally, a case-control study—as is true of any observational study—can identify only an association, or correlation, between two variables, and not a causation. This is an important distinction. An association merely shows that two events appear to be occurring together, not that one thing is causing the other. For example, shark attacks and ice cream sales both increase in the summer months but no one suggests that eating ice cream is causing an increase in shark attacks. So any headlines or news coverage implying that the current study concluded that the flu vaccine causes miscarriages were sadly (you might even say dangerously) misleading.

No study to date has found a causal link between receiving the flu vaccine and having a miscarriage. In fact, multiple previous studieshave shown not only that the flu vaccine is safe for pregnant women, but that it can save lives. This is because pregnant women are more prone to serious illness from flu (even requiring hospitalization in some cases).

The new study had two other weaknesses worth mentioning: It was relatively small in size, and the authors offered no theoretical explanation as to how the flu vaccine might lead to a miscarriage. It’s also worth noting that when the same researchers looked at data from two earlier flu seasons (2005-06 and 2006-07), they found no association between the flu vaccine and miscarriage.

Advice for pregnant women

The CDC and its Advisory Committee on Immunization Practices (ACIP) have not changed their recommendations for pregnant women. Pregnant women should receive the flu shot because the flu can be very dangerous for pregnant women (this is something we know with certainty). In addition, vaccinating the mother against the flu during pregnancy is the most effective way to protect the newborn baby from the flu, since infants younger than six months old can’t get the flu vaccine. In the aftermath of the new study, the American College of Obstetricians and Gynecologists has also reconfirmed its recommendationthat all pregnant women get vaccinated against the flu.

Bottom line: Pregnant women should be vaccinated against flu. The risks of influenza are far greater than the hypothetical risks of vaccination. Pregnant women who are concerned about the vaccine should discuss it with their doctors promptly. The vaccine is most effective when given before flu season begins, which means most people should get the vaccine before the end of October.

Also see Flu Shot: Does Time of Day Matter?