Flu vaccine during pregnancy may guard against stillbirths

5 Myths About the Flu Vaccine
5 Myths About the Flu Vaccine

(Reuters Health) - Women who get a seasonal flu vaccine during pregnancy may be much less likely to have a stillborn baby than unvaccinated mothers, a recent Australian study suggests.

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Researchers examined data on about 53,000 women who didn't get the vaccine during pregnancy and almost 5,100 who did. With the inoculation, women were 51 percent less likely to have a stillborn infant.

The risk of stillbirth is still quite small -- just 377 occurred in the entire study population, the equivalent of about 6.5 in every 1,000 births.

But the results suggest this inoculation may help babies thrive in the womb, said lead study author Annette Regan of the Western Australia Department of Health and the University of Western Australia in Perth.

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"Pregnant women should be reassured that flu vaccine does not increase the risk of poor birth outcomes for the baby, and in fact, getting a flu shot during pregnancy can reduce their risk of having a stillbirth," Regan said by email.

Scientists aren't certain how vaccination might protect against stillbirth, Regan added. Because flu during pregnancy is associated with stillbirths, however, it's likely that inoculation reduces the odds of flu infection for the mothers.

Even though doctors recommend that pregnant women get flu shots to protect themselves and their growing babies, less than half of mothers do, Regan and colleagues note in the journal Clinical Infectious Diseases. Often, women mistakenly fear the shots are unsafe for their babies.

When pregnant women get the flu, they're much more likely to have complications like preterm labor and premature births. If they get the flu early in pregnancy, it can lead to birth defects.

While pregnant women aren't supposed to get the inhaled vaccine FluMist (MedImmune), shots are safe during pregnancy.

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Pregnancy losses after at least 20 weeks of pregnancy were counted as stillbirths, not miscarriages. Pregnancy typically lasts about 40 weeks and babies born after 37 weeks are considered full term.

Overall, slightly less than 9 percent of the women in the study received flu shots during pregnancy.

Women were more likely to get flu shots if they were older and wealthier, or if they had pre-existing medical problems or developed complications like high blood pressure or diabetes during pregnancy.

Almost half of the vaccinated women got shots during the second trimester of pregnancy, while about 19 percent received inoculations sooner and 36 percent got shots later in pregnancy.

There were 5 stillbirths for every 100,000 combined days of pregnancy among women who didn't get vaccinated, compared with 3 stillbirths for every 100,000 pregnancy-days for vaccinated women.

The biggest reduction in stillbirths was seen for babies born just after the end of influenza season, when vaccination was linked to 67 percent lower odds of it happening.

One limitation of the study is that researchers relied on women's healthcare providers to report vaccinations to a government registry, which is optional, the authors note. This may have meant some women considered unvaccinated in the analysis actually did receive flu shots.

Researchers also didn't have enough data on vaccinated women in the study to determine how the timing of flu shots during pregnancy might influence the odds of stillbirth, the authors point out.

Stillbirths happen in less than 1 percent of pregnancies and are often caused by the placenta separating away from the uterine wall before delivery, a complication known as placental abruption, or by genetic abnormalities.

SOURCE: Clinical Infectious Diseases, online March 30, 2016.

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Originally published