Babies born to women who received a swine flu shot during pregnancy don’t have a higher risk of birth defects, Danish researchers found in one of the first studies targeting the safety of the vaccine on newborns.
The babies of mothers given the vaccine for H1N1 influenza virus, or swine flu, also weren’t any more likely than those of women who didn’t receive the vaccine to be born prematurely or smaller, according to research released today in the Journal of the American Medical Association.
Swine flu disproportionately affects pregnant women, causing severe complications including death, said Mark Steinhoff, who wrote an accompanying editorial in the journal. Today’s research and a Canadian study looking at the vaccine’s effect on Guillain-Barre syndrome, a disorder in which the body attacks its nervous system, shows the shot is safe and should be used as a precaution to prevent infection, he said.
“People are beginning to see there really are bad effects of flu,” said Steinhoff, director of the Children’s Global Health Center at Cincinnati Children’s Hospital in Ohio, in a July 6 telephone interview. “The vaccines are safe enough to recommend their use.”
The 2009 H1N1 virus was reported in more than 214 countries through August 2010, when the WHO declared an end to the pandemic. It probably killed about 284,500 people worldwide, researchers from the U.S. Centers for Disease Control and Prevention wrote in June in the journal Lancet Infectious Diseases. Swine flu has since become one of three seasonal flu strains circulating worldwide, causing infections mostly during the winter months.
Researchers in today’s study looked at 53,432 babies born in Denmark. Of those, 6,989, or 13 percent, had mothers who were given the flu vaccine during pregnancy. The Danish H1N1 vaccine is different from the U.S. vaccine because it contains a booster, called an adjuvant, that allows less of the vaccine to be used for the same result.
The researchers found a major birth defect was diagnosed in 18 of 330 infants whose moms were given the vaccine during the first three months of pregnancy compared with 15 of 330 in babies born to women not given the vaccine. The study also showed that pre-term birth occurred in 31 of the 300 infants exposed in the first trimester and 302 of 6,543 exposed in the second and third trimesters versus 24 of 330 in the first trimester and 295 of 6,366 in the second or third trimester.
“Our study contributes to the information available on the safety of influenza vaccination in pregnancy,” said study author Anders Hviid, a senior investigator in the Departments of Epidemiology Research at Statens Serum Institute in Copenhagen, in an e-mail. “It is by no means a complete evaluation of vaccination in pregnancy, and we cannot recommend specific courses of action based only on our study. However, our results are certainly reassuring to pregnant women and medical professionals.”
In a separate study, researchers said those in Quebec given the H1N1 vaccine in 2009 and early 2010 had a slightly higher risk of developing Guillain-Barre syndrome although the benefits of immunization likely outweigh the risks.
In Guillain-Barre syndrome, a person’s immune system damages nerve cells that can lead to muscle weakness and paralysis. Many things can cause the disease but most people who develop the syndrome do so after they’ve been sick with diarrhea or a respiratory illness, the U.S. Centers for Disease Control and Prevention said. It can also occur after a person has the flu and, on very rare occasions, it may develop after vaccination, the CDC said.
The researchers found about 2 cases of the disease for every 1 million doses given of the vaccine. Those ages 50 and older were at the highest risk, the authors said. That compares with the risk of hospitalization from a swine flu infection of 1 per 2,500 in Quebec, while the risk of death was 1 in 73,000.
“It does increase the risk but the risk is really quite small,” Steinhoff said. “The risk of Guillain-Barre is much smaller than the risk of being hospitalized with flu or having a very bad outcome.”
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