A new study published in The Journal of Infectious Diseases found that flu vaccine uptake among pregnant women, like the majority of the population, falls short of recommendations. But for pregnant women, skipping the flu shot or other antiviral treatment like oseltamivir (Tamiflu) could lead to pregnancy complications.

During the 2009 influenza pandemic, study authors found that pregnant women, despite representing 1 percent of the U.S. population, accounted for 6 percent of hospitalizations and 5 percent of deaths associated with severe flu. This form of flu is marked by intensive care unit admission, respiratory failure, and/or preterm delivery. It’s not just severe flu, though — the non-severe seasonal flu puts pregnant women at risk of hospitalization, too. But study authors found current estimates of vaccination coverage among pregnant women is around 50 percent.

The authors aimed to learn more about the epidemiology and clinical outcomes associated with flu hospitalizations among pregnant women in order to determine if early antiviral therapy (vaccine and oseltamivir), is beneficial. It's considered early if women seek treatment within two days of the onset of symptoms. Past studies have shown antiviral drugs to be safe and effective in pregnancy.

In their analysis, researchers included 3,169 pregnant women aged 15 to 44 years in the Influenza Hospitalization Surveillance Network, which includes over 240 hospital covering approximately 27 million people across 14 states. Data revealed 865 of these women were hospitalized within two days from a flu-positive test during the 2010-11 and 2013-2014 flu seasons. The authors defined hospitalization "as an admission to an inpatient ward of the hospital; an overnight stay was not required."

Women's medical records helped to categorize severe flu-related complications, from pneumonia, to respiratory failure, pulmonary embolism, asthma and COPD exacerbation, as well as dehydration. And after adjusting for underlying medical conditions, vaccination status, and pregnancy trimesters, the study authors found early antiviral therapy was associated with a shorter hospital stay.

Most hospitalized women had no underlying medical conditions and were admitted to the hospital for an average of 2.2 days if they had received treatment, which 26 percent had not; a majority of women had received oseltamivir. Women who did not or waited to seek treatment stayed in the hospital an average of 7.8 days.

Approximately 60 percent of women were in their third trimester at the time of hospitalization, and despite women being relatively healthy, 7 percent had severe flu, four women died, and another four experienced fetal loss. Of the 188 women who had a live birth during hospitalization, 41 were preterm deliveries. In fact, preterm delivery was more common among women with severe flu than women with non-severe flu.

"Treating pregnant women who have influenza with antiviral drugs can have substantial benefit in terms of reducing length of stay in the hospital," senior study author Dr. Sandra S. Chaves, of the Centers for Disease Control and Prevention (CDC), said in a press release. "The earlier you treat the better chances you have to modify the course of the illness."

Chaves and her team report their study is the first and largest to report on clinical characteristics and outcomes of pregnant women hospitalized with severe-flu since the 2009 pandemic. Vaccine uptake has improved since this time, but they believe coverage is "still suboptimal." Based on these findings, there should be a push for antiviral drugs in pregnancy. When speaking specifically to the flu vaccine, study authors found it reduced the risk of severe flu by approximately 50 percent among pregnant women.

The CDC "recommends treatment of suspected cases of flu among pregnant women with antiviral drugs as soon as possible, without waiting for test results to confirm influenza,” Chaves said. Annual flu vaccine reduces risk of associated complications for both women and their fetuses.

That said, these findings "should be tempered by recognizing some important limitations of the study." Drs. Alan T. N. Tita and William W. Andrews, of the University of Alabama at Birmingham, wrote in an accompanying editorial.

For one, the study does not assess the safety of oseltamivir in pregnancy, though current data does not indicate it poses risks to the fetus. In that same vein, Tita and Andrews said the study does not answer whether early treatment is associated with lower rates of important adverse outcomes, such as stillbirths and maternal deaths. The doctors also believe it would be helpful to know whether outcomes in the group who receive late treatment (after two days from symptom onset), are better if treated earlier.

"Overall, considering the accumulating evidence of fetal benefit and safety, influenza vaccination of pregnant and postpartum women should be a public health priority in accordance with national recommendations," Tita and Andrews concluded. "Prompt initiation of antiviral therapy if infection occurs, preferably within two days of suspected or confirmed influenza infection, is encouraged. Additional evaluation to assess fetal safety and to determine whether or not there is ongoing benefit when therapy is initiated after two days of symptoms are reasonable goals."

Source: Chaves S et al. The benefit of early influenza antiviral treatment of pregnant women hospitalized with laboratory-confirmed influenza. The Journal of Infectious Diseases. 2016.