A new study looking at whether women who are pregnant face an increased risk of a miscarriage if they get a flu shot found no link between the vaccine and pregnancy loss.
The reassuring finding contradicts an earlier study by the same researchers that raised questions about the safety of getting a flu shot during pregnancy. An overview of the study findings was presented Wednesday at a meeting of the Advisory Committee on Immunization Practices, which guides vaccination policy for the Centers for Disease Control and Prevention.
The new study was larger than the first and specifically designed to look at this question, leading the researchers to express confidence in the new findings. They will be submitting the data to a peer-reviewed journal for publication.
“For women right now who are wondering if it’s safe to get a vaccine in early pregnancy, we can say unequivocally, ‘Yes, it is safe,’” said Dr. Edward Belongia, who was part of the research team and who heads the Center for Clinical Epidemiology and Population Health at Wisconsin’s Marshfield Clinic.
This is the third time Belongia’s group has looked at the question. The first study, based on data from 2005-2006 and 2006-2007, found no association between flu shots and miscarriages.
After the 2009 pandemic, though, the CDC asked the group to study the question again. The reason: The virus that caused that pandemic, a new type of H1N1 virus, was incorporated into the annual flu shot for the first time in the 2010-2011 season. The CDC wanted to ensure that what was true before remained so.
To everyone’s surprise, the second study, which was published in 2017, raised a curious red flag. Some pregnant women who got flu shots did appear to be at higher risk of miscarrying, but only those who got vaccinated in the 28 days before their miscarriage — and in the previous year as well. Women who were only vaccinated in the year of their pregnancy didn’t appear to be at an increased risk.
All the studies were done using an observational design, meaning the researchers looked at data of pregnant women who chose on their own to be vaccinated or not. That type of a study cannot 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.
Observational studies have to be viewed with a degree of caution, because there could be unforeseen confounders — factors that influence the outcome. In the case of the earlier study, Belongia said it’s possible that the association that was spotted was actually false — a statistical fluke caused by some unrecognized confounding factor.
The earlier study was small, only 485 women. And the number of women in the group where the association seemed to occur was tiny — there were only 14 women who had been vaccinated two years running and had miscarried. When numbers are that small, one has to be careful in interpreting any findings.
The most recent study involved three times as many women, said James Donahue, who led the study.
“It is good news,” Donahue said of its finding.