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Trivalent flu vaccine works in pregnant women

NEW YORK (Reuters Health) – NEW YORK (Reuters Health) – The seasonal trivalent influenza vaccine (TIV) works as well in pregnant women as in the general population, a new study confirms.

The vaccine reduced the risk of acute respiratory illness (ARI) associated with laboratory-confirmed influenza among pregnant women by over one-half, which is on par with its effectiveness among all adults during the same seasons, the investigators report.

“Our findings should reassure women and their clinicians that the seasonal influenza vaccine appears to be at least as effective in preventing influenza illness among pregnant women as it is for other adults,” Dr. Mark Thompson from the Centers for Disease Control and Prevention in Atlanta, Georgia, told Reuters Health by email.

The TIV is recommended for all pregnant women in the U.S., but no studies in pregnant women have assessed laboratory-confirmed influenza outcomes, the researchers point out.

To help fill the gap, Dr. Thompson and colleagues did a case-control study over the 2010-2011 and 2011-2012 influenza seasons among Kaiser Permanente health plan members in two metropolitan areas in California and Oregon.

They compared the proportion vaccinated among 100 pregnant women with influenza (confirmed by RT-PCR) with the proportions vaccinated among 192 controls with ARI who tested negative for influenza and 200 controls without ARI.

“This approach, often referred to as the test-negative design, is believed to minimize potential bias and confounding by the propensity to seek health care and has been employed in other studies of vaccine effectiveness (VE) among adults during the same seasons,” the investigators explain in Clinical Infectious Diseases, online November 26.

Among the 100 influenza cases, 42% were vaccinated during the study season compared to 58% vaccinated among influenza-negative controls and 63% vaccinated among matched ARI-negative controls.

The adjusted VE was 44% using the influenza-negative controls and 53% using the ARI-negative controls. “Precision of VE estimates by season and (sub)type were limited by the small number of cases,” the investigators point out.

Using influenza-negative controls, the adjusted VE point estimate was higher in the 2010-2011 season (57%) than in the 2011-2012 season (27%). VE estimates using influenza-negative controls were similar for A(H1N1)pdm09 virus infection (57%), A(H3N2) virus infection (41%), and influenza B infection (46%).

Similar findings were observed using the matched ARI-negative controls.

In secondary analyses, the researchers observed that vaccination with the prior season’s vaccine had an effect similar to receipt of the current season’s vaccine. Vaccination in either or both seasons had statistically similar adjusted VE using influenza-negative controls (VE point estimates range, 51%-76%) and ARI-negative controls (48%-76%).

“The similar protection conferred by influenza vaccination in either the current or prior season in our study should not be interpreted as indicating that a pregnant woman can forgo vaccination if she were vaccinated in the previous season,” the investigators emphasize.

Summing up, they say this study confirms “substantial and significant” effects of influenza vaccination in pregnant women.

“Taken together with other recent studies, influenza vaccines appear to be not only safe for mothers and fetuses, but equally as effective as in non-pregnant adults. As pregnant women are at risk for serious influenza-related outcomes, our results support the recent WHO recommendation that all countries with influenza vaccination programs vaccinate pregnant women,” they conclude.

Dr. Thompson said, “We need to continue efforts to increase awareness of the risks of influenza during pregnancy and both the safety and benefits of the influenza vaccine for pregnant women as well as the potential secondary protection passed on to their newborns.”

Commenting on the study for Reuters Health, Dr. Andy Pavia, a member of the influenza advisory group of the Infectious Diseases Society of America, said it provides “clear confirmation that influenza vaccine works in pregnant women. That was not a given because the immune system of pregnant women changes, especially in the last trimester.”

“We have known for a long time that influenza can be more serious in pregnant women. The increased risk for pregnant women became abundantly clear during the 2009 pandemic, resulting in more attention directed towards diagnosing influenza in pregnant women and starting influenza antivirals promptly. Because of the higher risk of severe illness, hospitalization and death among pregnant women who develop influenza, we have recommended influenza vaccine for them.” Dr. Pavia said.

“Clinicians should realize that pregnant women are at higher risk of complications of influenza and the influenza vaccine will decrease the risk of symptomatic influenza by about one half. We also know from many years of using influenza vaccine and tracking safety that the vaccine is safe in pregnancy. In addition, when a pregnant woman is vaccinated against influenza, her infant is protected from influenza infection and hospitalization because of the mother’s antibodies that are passed across the placenta. Therefore, clinicians should strongly recommend influenza vaccine for their pregnant patients,” Dr. Pavia said.

The fact that influenza vaccination rates in pregnant women in the US have risen to 50% is “pretty encouraging,” Dr. Pavia added. He noted that before 2009, “little attention” was given to influenza in pregnancy, so to have increased from close to 0% to almost 50% is a “great start.”

“Obstetricians had not been in the business of recommending or giving vaccines to their pregnant patients and the general message had been to avoid most medications and vaccines during pregnancy. We still have a ways to go in communicating the benefits of influenza vaccine to pregnant women and developing ways to make it easy for them to be vaccinated,” Dr. Pavia said.


Clin Infect Dis 2013.