WebMD Medical News
Laura J. Martin, MD
Oct. 20, 2011 -- Pregnant women are more likely to get a flu shot than they were a few years ago, and for good reason: Evidence continues to mount that the vaccine safely protects both mother and newborn from influenza and its complications.
One new study shows that pregnant women who get a flu shot are no more likely to miscarry. A second, small study shows that babies of moms who received the vaccine retain some immunity for two months after birth.
In a third study, more than half of the pregnant women surveyed were immunized in the 2010-2011 flu season. Historically, the figure has hovered at a dismal 15%, researchers say.
"We're building a large and consistent body of evidence regarding the benefits and safety of flu vaccination in pregnancy," says Kathleen Neuzil, MD, MPH. She is a member of the Infectious Diseases Society of America's (IDSA) pandemic influenza task force and director of the influenza vaccine project at PATH, a nonprofit agency aimed at improving global health. Neuzil was not involved with any of the new research.
Past research has shown that pregnant women who are immunized are less likely to have babies who are premature, develop respiratory disease, or need to be hospitalized, she tells WebMD.
Neuzil and other experts meeting at the annual IDSA meeting in Boston say they hope the new findings will convince more pregnant women to get the flu vaccine.
Marci L. Drees, MD, chief epidemiologist at Christiana Health Care System in Newark, Del., and colleagues surveyed about 300 women who had just given birth during the 2009-2010 and 2010-2011 flu seasons.
During the 2009-2010 season, 191 of 307 (62%) said they had received the H1N1 swine flu vaccine and 186 (61%) reported they got the seasonal flu shot during pregnancy.
In 2010-2011, 165 (55%) of 300 women surveyed said they received the seasonal flu vaccine.
Drees tells WebMD she "was pretty happy" vaccination rates didn't drop even further last year.
"In 2009-2010, there was so much attention paid to the H1N1 pandemic," she says. "This past year was not as exciting of a flu season. People forgot how sick unvaccinated people were the year before."
Still, nearly half of pregnant women in the study didn't get vaccinated last year. "We need to do better," Drees says.
One way to do that, she says, is to ensure all ob-gyns offer their patients flu shots.
Other findings from the study:
A second study, conducted at the University of Utah, confirmed that pregnant women who get the seasonal flu vaccine pass their immunity to their babies and that the protection lasts for two months after birth.
The researchers studied 27 women, 11 (41%) of whom received the seasonal influenza vaccine. All babies born to immunized women had flu antibodies in their blood at birth, compared to 31% of babies born to women who had not received the vaccine.
At two months, 60% of babies born to immunized women had antibody protection vs. none of the babies born to women who weren't vaccinated.
By four months, however, there was no difference in antibody rates between the two groups.
But wouldn't it have been better to compare the rates of flu and flu complications among the two groups than antibody rates?
Yes, says study researcher Julie H. Shakib, DO, MPH, assistant professor of pediatrics at the University of Utah in Salt Lake City. "But we need a much larger study to do that."
On the other hand, antibody rates are a reasonable substitute for flu rates, Shakib and Neuzil agree. Other research has shown an association between the two, they explain.
The third study, the largest to date on the topic, showed there is no association between getting the flu vaccine during the first trimester of pregnancy and miscarriage.
Researchers at the Marshfield Clinic Research Foundation in Marshfield, Wis., compared the medical records of 243 pregnant women who had a miscarriage and 243 who did not.
A total of 38 women (16%) who miscarried had received the influenza vaccine in the four weeks before the pregnancy loss, compared to 31 women (13%) who did not miscarry. That's a difference so small it could be due to chance.
"Although there was never any hard evidence that influenza vaccine can cause pregnancy loss, there have been fears about it," study researcher Stephanie A. Irving, MS, an epidemiologist at Marshfield, tells WebMD.
"This study should provide reassurance," she says.
These findings were presented at a medical conference. They should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.
SOURCES:49th Annual Meeting of the Infectious Diseases Society of America, Boston, Oct. 20-24, 2011.Kathleen Neuzil, MD, MPH, director, influenza vaccine project, PATH.Marci L. Drees, MD, chief epidemiologist, Christiana Health Care System, Newark, Del.Julie H. Shakib, DO, MPH, assistant professor of pediatrics, University of Utah, Salt Lake City.Stephanie A. Irving, MS, epidemiologist, Marshfield Clinic Research Foundation, Marshfield, Wis.
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