News Release: Research

Oct. 29,  2009

Studies Show Flu Vaccines Safe in Pregnancy and Risks of Disease Complications Higher

Pregnant Women Should Be Encouraged to Receive H1N1 Vaccine, Say Researchers

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Pregnant women are at particularly high risk of serious disease and death after flu infection, and healthcare providers should encourage them to be vaccinated, say a team of vaccine and public health researchers.

The researchers reviewed past studies of flu vaccination during pregnancy, as well as data about disease and death from flu infection. No study showed an increased risk of maternal or fetal complications associated with inactivated flu vaccination, but data from previous pandemics and seasonal epidemics confirmed that pregnant women are at increased risk of serious complications from flu. All the evidence indicates a clear recommendation for vaccination, they say.

The review article is published online in the American Journal of Obstetrics & Gynecology.

The research team included collaborators at Emory University, Johns Hopkins University and Cincinnati Children's Hospital Medical Center. Saad B. Omer, MBBS, MPH, PhD, assistant professor of global health at Emory's Rollins School of Public Health, was senior author and Pranita D. Tamma, MD, of the Division of Pediatric Infectious Disease at Johns Hopkins Children's Center, was lead author.

"Healthcare providers will play a key role in women's decisions about whether or not to be vaccinated against H1N1", says Omer. "There is substantial evidence that vaccination is not only safe for pregnant women but that it is critical for protecting women and their infants against serious complications from the flu. Physicians and other providers should talk about risks and benefits with their patients and help alleviate any unfounded fears."

The researchers reviewed 11 published research studies on safety outcomes from vaccination between 1964 and 2008. These studies showed no increased risk of either maternal complications or adverse fetal or newborn outcomes associated with inactivated flu vaccination.

They also reviewed morbidity and mortality data on pregnant women and newborns from the Centers for Disease Control and Prevention (CDC) from past flu epidemics. The CDC data show that in spring 2009, 32 percent of the 34 pregnant women with confirmed or probable cases of H1N1 had to be hospitalized, and pregnant women accounted for 13 percent of all deaths from H1N1 during that time. During the 1918 pandemic, half of infected pregnant women developed pneumonia, and of these, almost half died. During the pandemic of 1957, nearly half of all women of childbearing age who died were pregnant. Most of the women who died during these outbreaks were healthy before being infected with influenza.

"During seasonal influenza epidemics, previous pandemics, and with the current influenza A (H1N1) pandemic, pregnancy places otherwise healthy women at increased risk for serious complications from influenza, including death. Health-care workers caring for pregnant females can play a pivotal role in helping to protect women and newborns from this vaccine-preventable disease and should anticipate questions that expecting mothers may have regarding vaccine safety," says Tamma.

Since 2004 the CDC Advisory Committee on Immunization Practices (ACIP) has recommended influenza vaccination for all women who are or will be pregnant during the flu season. The American College of Obstetricians and Gynecologists says flu vaccination is an essential element of prenatal care. Pregnant women are in the highest-priority group recommended by the CDC to receive the current H1N1 vaccine.

Despite these recommendations, however, and evidence that pregnant women are at increased risk of death from the flu, pregnant women have historically had the lowest rate of vaccination of all adults recommended to receive seasonal flu vaccination.

Although pregnant women do not have a higher incidence of flu than other women, changes in the immune system during pregnancy contribute to women's increased risk for serious flu-related complications. One previous study shows that healthy pregnant women are 18 times more likely to be hospitalized from flu than healthy non-pregnant women. And pregnant women with other medical problems are at even greater risk.

Other studies have shown that flu vaccination during pregnancy not only protects mothers, but also evokes an antibody response that transfers to the developing fetus and protects very young infants.

Studies also have addressed safety concerns about the use of thimerosal in vaccines. The research team reviewed several studies that showed no causal link between thimerosal in vaccines and complications in infants. Thimerosal-free versions of H1N1 vaccine will be available this fall, but the ACIP recommends vaccination even if these vaccines are not available.

Other members of the research team included Kevin A. Ault, MD, professor of gynecology and obstetrics, Emory University School of Medicine; Carlos del Rio, MD, professor and chair in the Hubert Department of Global Health, Rollins School of Public Health, Emory University; Mark C. Steinhoff, Cincinnati Children's Hospital Medical Center; and Neal A. Halsey, Johns Hopkins University.

Reference: Safety of influenza vaccination during pregnancy. Tamma et. al. American Journal of Obstetrics & Gynecology

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