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Although a 2010-12 study suggested a heightened risk of miscarriage after the shot, experts have yet to conclusively link the two.shironosov/Getty Images/iStockphoto

The question

I am pregnant and my doctor says I should get a flu shot to protect my unborn child and myself against influenza this winter. But I read a news story about a study that found the flu vaccine might be linked to an increased risk of miscarriages in pregnant women. Is this risk for real?

The answer

Even the researchers who did the study, using 2010-12 data, aren't sure if the risk is real or if some error or statistical fluke can explain the link to miscarriages.

"I was surprised by the findings," says Jim Donahue, the lead author of the study and an epidemiologist at the Marshfield Clinic Research Institute (MCRI) in Wisconsin.

In fact, he was involved in an earlier study that found no association between the vaccine and miscarriages during the 2005-06 and 2006-07 flu seasons.

That earlier study, which was funded by the U.S. Centers for Disease Control and Prevention (CDC), was part of a continuing effort by the U.S. medical community to monitor the safety and effectiveness of vaccines.

The need for the latest study arose in 2009 after the emergence of a pandemic flu strain known as H1N1. A pandemic strain represents such a significant change in a virus that most people have had no prior exposure to anything like it. Without immunity in the population, a pandemic virus is capable of causing above-average levels of severe sickness and death.

In the following flu season, the annual influenza vaccine was modified to protect the public against H1N1. This was the first pandemic flu strain in four decades. So, the CDC asked Donahue and his colleagues to repeat their miscarriage study just to be sure the new flu vaccine was as safe as earlier versions.

The first thing you need to know is that this is not an easy study to do.

The most reliable scientific evidence comes from randomized-controlled trials in which one group of patients receives a particular treatment and other patients do not. Data from the two groups are analyzed to see if the patients' health outcomes are different.

But researchers can't ethically withhold something as essential as a flu shot, which is recommended for all pregnant women. As a result, they must rely on indirect ways to investigate – drawing inferences from a sample of the population.

In this study and the earlier one, they used a case-control approach. In particular, they searched the medical records of several U.S. health-care organizations to identify women who had miscarriages between 2010 and 2012. They then looked for comparable women – or "controls" – who had full-term pregnancies during the same time period.

In total, the researchers selected 485 cases and 485 controls.

When the two groups of women were compared, the results suggested that those who had received flu shots for two years in a row appeared to be at an elevated risk of having a miscarriage – also known as a spontaneous abortion.

The researchers are quick to point out that their findings, which are based on a relatively small sample, don't prove that flu shots cause miscarriages.

"You almost never get a definitive answer with just one observational study," Donahue says.

Nor do the researchers have a plausible explanation for why the vaccine might trigger a miscarriage – something that has never been observed before. "We don't know what the mechanism might be," says Edward Belongia, the senior author of the study and director of the Center for Clinical Epidemiology and Population Health at MCRI.

He says it's possible the vaccine produced some type of heightened immune response, similar to a booster shot, for those who received the vaccine in two consecutive flu seasons.

Despite the inconclusive nature of their findings, "we have an obligation to the public to be completely open and transparent about what we know and what we don't know," Belongia says. Their study was published in September in the journal Vaccine.

The U.S researchers have already started on a larger follow-up study covering recent flu seasons. Those results are expected to be available later next year.

In the meantime, public health officials continue to recommend that pregnant women get the vaccine for the coming flu season. There is simply not enough evidence to warrant any change.

What is known for certain is that the flu can harm pregnant women – seriously enough to land them in hospital and cause death. The flu poses a major threat to fetuses, too.

"During pregnancy, influenza infection itself is associated with spontaneous abortion, premature delivery, as well as inadequate fetal growth," says Andrew Simor, chief of microbiology and infectious diseases at Sunnybrook Health Sciences Centre in Toronto.

A vaccination causes the body to produce infection-fighting antibodies. When a pregnant woman receives a flu shot, her antibodies get passed on to the developing fetus, providing protection to the newborn in the first months of life. A baby can also acquire antibodies through a vaccinated mother's breast milk.

"The benefits of vaccination are enormous," says Simor, adding that many previous studies have found flu shots to be safe in pregnancy.

If further research finds that the U.S. miscarriage study did identify a real risk, it could be a time-limited reaction associated with the introduction of H1N1 components into the vaccine.

Why time-limited? A pandemic flu will morph into a regular seasonal virus as more and more people are exposed to it and build up immunity. In a similar fashion, a potentially negative response to the H1N1 component in the vaccine may diminish through repeated exposure, Belongia speculates.

"We wish we had more definitive answers, but we will get this sorted out," he adds.

Paul Taylor is a patient navigation adviser at Sunnybrook Health Sciences Centre. He is a former health editor of The Globe and Mail. You can find him on Twitter @epaultaylor and online at Sunnybrook's Your Health Matters.

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