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Study raises questions about flu shots during pregnancy

Flu shot vaccine (MGN Online file photo) 

A perplexing new study on flu shots during pregnancy is raising concerns among expectant mothers about the risk of miscarriage. But doctors - and even the study's authors - caution that more research is needed.

The study found that women who had miscarriages between the years 2010 and 2012 were more likely to have had back-to-back annual flu shots that included protection against swine flu.

Vaccine experts believe it is possible those miscarriages did not result from the vaccinations but the older age of those women and other risk factors.

Past studies have found flu vaccines are safe during pregnancy, but there's been little research on their impact during the first trimester.

Before these new findings were published, the Centers for Disease Control (CDC) notified the American Congress of Obstetricians and Gynecologists (ACOG), anticipating a lot of questions, confusion and worry.

“We certainly want to montior the safety of vaccines. But it clearly needs more research before any significant changes in clinical practice should be recommended,” said ACOG Vice President for Practice Dr. Christopher M. Zahn. “The risk of serious illness – the risk of death – from flu fortunately is not very common, but it does occur. And it's completely avoidable with the vaccine.”

How would Zahn counsel a pregnant family member seeking his advice on whether to get a flu shot?

“I think it's important that women know all the facts and have all the information in order to make those decisions,” he said. “But I would definitely advise women of the benefits of the flu vaccine and recommend that she have it done.”

Flu and its complications kill thousands of Americans every year. The elderly, young children and pregnant women are especially at risk. When a new "swine flu" strain emerged in 2009, it killed 56 U.S. pregnant women that year, according to the CDC.

The study's authors, two of whom are CDC researchers, saw a big difference when they looked at women who had miscarried within 28 days of getting a shot that included protection against swine flu, but it was only when the women also had a flu shot the previous season.

They found 17 of 485 miscarriages they studied involved women whose vaccinations followed that pattern. Just four of a comparable 485 healthy pregnancies involved women who were vaccinated that way.

The first group also had more women who were at higher risk for miscarriage, like older moms and smokers and those with diabetes. The researchers tried to make statistical adjustments to level out some of those differences but some researchers don't think they completely succeeded.

Other experts said they don't believe a shot made from a killed flu virus could trigger an immune system response severe enough to prompt a miscarriage. And the authors said they couldn't rule out the possibility that exposure to swine flu itself was a factor in some miscarriages.

Two other medical journals rejected the article before a third, Vaccine, accepted it. Dr. Gregory Poland, Vaccine's editor-in-chief, said it was a well-designed study that raised a question that shouldn't be ignored. But he doesn't believe flu shots caused the miscarriages. "Not at all," said Poland, who also is director of vaccine research at the Mayo Clinic.

Though this study may cause worry and confusion, it is evidence "of just how rigorous and principled our vaccine safety monitoring system is," said Jason Schwartz, a Yale University vaccine policy expert.

Some of the same researchers are working on a larger study looking at more recent data to see if a possible link between swine flu vaccine and miscarriage holds up, said James Donahue, a study author from the Wisconsin-based Marshfield Clinic Research Institute. The results aren't expected until next year at the earliest, he said.

The Associated Press contributed to this story

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