Flu Shot Protects Mom and Unborn Baby

Pregnant moms receiving flu shots had lower rate of miscarriages during pandemic

/ Author:  / Reviewed by: Robert Carlson, M.D

(RxWiki News) With a flu pandemic going on, public health authorities are encouraging everyone to get flu shots. But pregnant women may worry – will the shot hurt my baby?

The answer is no. In fact, a flu shot may even offer a bit of extra protection to pregnant women's unborn babies, even beyond protection from flu infection.

However, a flu infection in a pregnant woman is a serious concern and can harm a developing baby.

These are the findings of a recent study that aimed to understand the possible risks to an unborn baby of the flu vaccine and a flu infection.

"Get your flu shot."

The study, led by Siri E. Håberg, MD, PhD, from the Norwegian Institute of Public Health, involved looking at all the pregnancies in Norway from 2009 through 2010.

Of 117,347 pregnancies, the miscarriage rate of babies past the 12th week of pregnancy was 4.9 fetuses per 1,000 births. Among the 113,331 pregnancies of single babies only (or of mother's with uncertain vaccination status), the rate was 4.3 miscarriages per 1,000 births.

However, when the researchers looked specifically at vaccinated and unvaccinated pregnant women during the 2009 influenza A (H1N1) pandemic months, a pattern emerged.

Of the 25,976 pregnant women who got the flu shot, 78 had miscarriages during the flu pandemic, for a rate of three out of every 1,000 babies. Among the 87,335 unvaccinated pregnant women, there were 414 miscarriages during the pandemic, for a rate of 4.7 out of every 1,000 babies.

A total of 54 percent of the women in their second or third trimester of pregnancy received the flu shot. During the pandemic, women who did not get the flu shot were 25 percent more likely to lose their baby than those who did get the flu shot.

Meanwhile, among pregnant women who were actually diagnosed with influenza, the rate of unborn babies' deaths was almost double that of women who did not have the flu. Of the 2,795 women who had a clinical or lab diagnosis of influenza, 21 lost their babies, for a rate of 7.5 miscarriages out of every 1,000 babies.

The researchers also found that women who received the flu shot were slightly (about 12 percent) less likely to have their baby die in the womb. However, it's possible this was due to chance.

The scientists could not say the slightly better rate of baby survival in vaccinated moms was directly due to the flu shot because of other possible factors and because the difference in outcomes was so small between those who did and didn't get the shot.

Overall, however, the researchers noted that the evidence favors pregnant women getting flu shots.

"Given the danger posed by maternal influenza virus infection for fetal survival, our study adds to the growing evidence that vaccination of pregnant women during an influenza pandemic does not harm – and may benefit – the fetus," the authors wrote.

"We found no basis for withholding influenza vaccination from pregnant women in their second or third trimester – an important group, given that these women can be particularly vulnerable to the severe effects of influenza virus infection," they added.

The researchers noted that it's possible women with already known risk factors for miscarriage, such as diabetes or a history of miscarriage, may have been more or less likely to get the flu shot. This could have a small effect in their findings.

They also found that women with chronic conditions and obesity were more likely to get the flu shot whereas smokers were less likely – another factor that could have a slight impact on the findings.

Overall, however, when the researchers made adjustments to try to account for these differences, it did not alter the overall finding that a flu infection was linked to a higher miscarriage rate and getting a flu shot was linked to a lower miscarriage rate.

The study was published January 16 in the New England Journal of Medicine. The research was funded by the Norwegian Institute of Public Health. One author received funding from the Intramural Research Program of the National Institute of Environmental Health Sciences at the US National Institutes of Health.

Reviewed by: 
Review Date: 
January 14, 2013
Last Updated:
August 19, 2013