May 11, 2012 (Baltimore, Maryland) — A review of reports to the Vaccine Adverse Event Reporting System (VAERS) has confirmed that flu vaccines are safe for pregnant women, according to the results of a study presented here at the National Foundation for Infectious Disease 15th Annual Conference on Vaccine Research.

The review encompassed 22 years (1990 to 2012) and millions of women receiving flu vaccinations. "It did not reveal any concerning or unusual clinical patterns among pregnant women or their infants," said lead investigator Karen Broder, MD, medical officer at the Immunization Safety Office, US Centers for Disease Control and Prevention (CDC), in Atlanta, Georgia.

Since 1997, the Advisory Committee on Immunization Practices has routinely recommended trivalent inactivated influenza vaccine (TIV) for pregnant women.

Pregnant women were a priority group for vaccination with monovalent H1N1 inactivated influenza vaccine during the 2009/10 pandemic because they are at high risk for morbidity from seasonal and pandemic flu.

To describe the adverse events that pregnant women who received inactivated influenza vaccines experienced, researchers looked at the data from VAERS, the nation's early-warning system for adverse events. Researchers examined surveillance reports for seasonal TIV (many strains) from 1990 to 2009; the 2009 monovalent H1N1 influenza vaccine (2009/10 pandemic); and the seasonal TIV (including the 2009 H1N1 strain) from 2010 to 2012.

They also performed an automated search for pregnancy coding terms, requested medical records, and performed clinical reviews of reports and any records that referenced pregnancy. As a caveat, Dr. Broder noted that VAERS, which is a passive surveillance system, is not designed to assess whether a vaccine actually caused an adverse event.

From 1990 to 2009, 118 adverse-event reports were made after TIV administration. During the 2009/10 flu pandemic, the number of adverse-event reports surged to 288. "This was not an unexpected finding because of the high level of publicity about the pandemic and about vaccine safety," Dr. Broder explained.

After the pandemic — from 2010 to 2012 — the number of reports was higher than before, but dropped from the high point in 2009/10 to 130. In most cases, the flu vaccine was the only vaccine administered.

The median interval from vaccination to the beginning of an adverse event ranged from 1 to 2 days. Two maternal deaths occurred during the 2009/10 season. One involved a woman with a ruptured aortic aneurysm; the other was a mother who suffered hemorrhagic shock after cesarean delivery.

The most common pregnancy-specific adverse event was spontaneous abortion, which is not surprising, said Dr. Broder, because it's a relatively common adverse event in general for pregnant women. The next most common event was a stillbirth.

About half of the verified spontaneous abortion reports had at least 1 risk factor, and 72% of stillbirth reports were linked to at least 1 risk factor. The most common risk factor was maternal age of at least 35 years.

A review of more than 530 reports revealed that the most common trimester for vaccination shifted a little with early reports (1990 to 2009), when the most common exposure was during the second trimester. Later reports were linked to vaccination in the first or first and second trimesters.

The most common nonpregnancy-specific adverse events were injection-site reactions and nonanaphylaxis allergic reactions. "Again, this is not unexpected," said Dr. Broder. Seven congenital anomalies were identified, but researchers did not find any pattern that merited concern.

One audience member asked about the much higher proportion of spontaneous abortions during the pandemic. Dr. Broder explained that the numbers likely reflect better reporting and more recognition of vaccine safety. The background rate was about 49 per 1,000,000 pregnant women vaccinated, which still is many orders of magnitude below what you would expect from pregnant women. According to Dr. Broder, spontaneous abortions happen in 10% to 22% of pregnancies. "We think this was largely an effect of stimulated reporting," she said.

"These findings affirm the safety of influenza vaccine during pregnancy," moderator Georges Peter, MD, professor emeritus at Warren Alpert Medical School of Brown University in Providence, Rhode Island, told Medscape Medical News.

One could say that the adverse event for a pregnant woman who does not get a flu vaccine is the potential for more serious influenza. One of the risk factors for serious outcomes in the 2009/10 pandemic, Dr. Peter noted, was pregnancy among unvaccinated women. "I think these findings are very reassuring to women," Dr. Peter said. "I think they also indicate the continuing commitment to our immunization safety office at the CDC to follow safety, and not simply to accept the studies that lead to licensure."

Dr. Broder and Dr. Peter have disclosed no relevant financial relationships.

National Foundation for Infectious Diseases (NFID) 15th Annual Conference on Vaccine Research: Session S30. Presented May 9, 2011.