A new study has found that flu shots do not cause miscarriages, in spite of one scientific study and ample speculation positing a connection.
As CNN reports, the new study was presented last week to the U.S. Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices. The CDC had asked two researchers — Edward Belongia, director of the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic Research Institute in Wisconsin, and James Donahue, a senior epidemiologist at Marshfield — to further investigate a study they conducted during the 2010-11 and 2011-12 flu seasons. That study found an increased risk of miscarriage in the 28 days after vaccination—but only among women who had received an H1N1 flu vaccine two years in a row.
As STAT points out, the study was observational, meaning that it was based on women who had opted on their own to receive or decline a flu shot, and therefore the research cannot definitively determine causation. “It can only deduce that something—in this case a flu shot early in pregnancy—appeared to have been linked to an elevated risk of miscarriage,” STAT writes.
The study was also relatively small, drawing on data from only 485 people, and just 14 women who had been vaccinated two years in a row experienced miscarriages. The more recent study is much larger. It looked at the 2012-13, 2013-14 and 2014-15 flu seasons, both individually and together, and involved 1,236 pairs of women. In each pair, one woman had miscarried between 6 and 19 weeks, and the other had delivered a baby. The women were also paired off based on whether or not they had gotten the flu vaccine the previous season; 627 pairs had been vaccinated, and 609 pairs had not.
The researchers found no increased risk of miscarriage after vaccination, prompting them to chalk the results of the previous study up to “minor variations [that] could be explained by chance,” Belongia tells CNN. This in turn means that medical professionals should continue to recommend that pregnant women receive a 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,” Donahue tells CNN. “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.”
Read more at CNN.