New Research Has Concluded The Flu Shot Won't Cause Miscarriages

In 2012, the Centers for Disease Control and Prevention shared an alarming study about vaccination. It appeared as though there might be some risk that a pregnant woman receiving the flu shot may suffer a miscarriage. At the time, the study didn't seem to follow along with any previous scientific evidence regarding flu shots and pregnancy, but the CDC wanted to investigate further with a more comprehensive study. That study has now finally concluded, as CNN reported this week, and researchers found that the flu shot won't cause miscarriages.

The CDC-funded study from the 2011-2012 season reported that pregnant women who were vaccinated with the H1N1 (H1N1pdm09) one year and had been vaccinated with the H1N1pdm09-containing flu vaccine the previous year were at an increased risk for miscarriage within 28 days after receiving the flu shot, generally in the first trimester but sometimes up to the second trimester, according to the American Academy of Family Physicians.

This was obviously disconcerting news, considering the flu shot could potentially protect pregnant women and their babies from viral illnesses, like the flu, when they are particularly vulnerable.

While the CDC shared the information, the health agency was skeptical for a few reasons. As noted on the CDC's website: "Earlier studies have not found a link between flu vaccination and miscarriage. There is an ongoing investigation to study this issue further among women who were pregnant and eligible to receive flu vaccine during the 2012-13 through 2014-15 flu seasons. Results are anticipated in late 2018 or 2019." And those results, released on Wednesday, Feb. 27, are definitively in.

According to CBS News affiliate WTYV, the very same researchers who worked on the first study were in charge of the follow-up — Marshfield senior epidemiologist James Donahue and Dr. Edward Belongia, director of the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic Research Institute in Wisconsin.

The first study had some issues that had even the two lead investigators questioning their results. According to STAT News, it was a small group and they had no knowledge of pre-existing conditions that could make miscarriages more likely.

As Donahue explained to CNN, "Those results were unexpected, and we weren't convinced they represented a causal association. So we designed another study that was much larger to investigate the unusual finding."

The new study looked at 1,236 pairs of women — 627 pairs who were vaccinated last season and 609 who weren't. This study population was three times the size of the first, allowing them a much more comprehensive picture.

And here is what they found — no risk of miscarriage due to flu shots. Even when they looked at all of the flu seasons and all of the women in the study combined, according to the CDC, they were able to conclude that the flu shot did not increase risk of miscarriage.

In fact, Donahue told CNN that quite the opposite is true; pregnant women should absolutely be getting the flu shot. "There's lots of evidence of the severity of flu for a pregnant woman, more chance of hospitalization, more risk of death, especially as she enters the second and third trimester," he explained to the news outlet. "There are also many studies that show the mother's vaccination will help protect the newborn baby from flu, which is critical since the baby cannot be vaccinated until 6 months of age."

The Mayo Clinic also recommends that pregnant women get the flu shot since it can help to prevent possible flue-related fetal and maternal complications. Pregnancy changes a mother's immune system and getting the flu shot offers an optimum chance to protect your baby as well as yourself, as Live Science explained.

This new research should help to ease the concerns of some pregnant women who were still unclear about getting the flu shot. And with any luck, the more comprehensive study will discourage any more anti-vaccine misinformation from spreading.