Dear Doctor: I normally get a flu shot every year, but now I'm pregnant and have read that the vaccine might cause miscarriages. Is this true?

Dear Reader: First, let's explain why this is an issue: Pregnant women who contract influenza are more likely to develop complications from the influenza virus. The most striking example of this occurred during the H1N1 pandemic in 2009, when pregnant women accounted for 5 percent of virus-related deaths, but comprised only 1 percent of the population.

In general, pregnant women with influenza are more likely to require hospitalization and more likely to end up in the intensive care unit. They also have a greater likelihood of giving birth to children with cleft lip, neural tube defects (spina bifida), hydrocephalus (increased fluid within the brain) and heart defects. These complications of influenza led the Centers for Disease Control and Prevention in 2004 to recommend that all pregnant women get the influenza vaccine, regardless of trimester.

Now let's look at the safety of the vaccine itself.

A recent study in the journal Vaccine obtained data from health care organizations in five states about women who were pregnant during the 2010-2011 and 2011-2012 flu seasons. The authors determined which ones had been vaccinated with the standard influenza vaccine and which ones suffered a spontaneous abortion, or miscarriage. They found that 485 women had a miscarriage; they then compared these women to 485 pregnant women who didn't have a miscarriage.

The major difference was seen among women who were vaccinated the year before they were pregnant and then repeated the vaccination when they were pregnant. Of those, 17 women in the miscarriage group had the vaccine within 28 days prior to the miscarriage. Women in the group who didn't have miscarriages were five times less likely to have had the flu vaccine in that preceding 28 days.

It's possible that an inflammatory response could be to blame for the miscarriages. Because the first shot primes the immune system, the second shot gives a greater immune response, leading to inflammation. Excessive inflammation can then lead to a spontaneous abortion.

However, the authors stressed that their study found a correlation, not a causation. Here are some other factors that emphasize the far-from-conclusive nature of the study:

First, the number of women in this part of the study was small: 17 in the miscarriage group and four in a group who didn't have a miscarriage at the same time. Second, women at increased risk of miscarriage may be more likely to seek medical help and thus be more likely to get a flu shot.

Obviously, a larger study is needed to verify -- or repudiate -- the possible connection. But, ethically, this may be difficult. A researcher would have to compare pregnant women who had the flu vaccine the year before, and give one group the flu shot in the first weeks of pregnancy and not the other. Yet the risks of going unvaccinated are simply too great.

Until we have further data, I would recommend flu shots in pregnancy to prevent the risk of maternal death and birth defects.

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