Influenza A H1N1: Disease more severe in pregnant women

Officials project that the disease may affect up to 40% of the U.S. population this influenza season.

Higher rates of hospitalization and a greater risk for death are more common among pregnant women with novel influenza A H1N1 compared with the general population, health officials said.

“Some clinicians hesitate to treat pregnant women with antiviral medications because of concerns for the developing fetus, but this is the wrong approach,” Denise Jamieson, MD, MPH, clinical associate professor in the department of gynecology and obstetrics at Emory University, Atlanta, and a medical officer in the CDC’s U.S. Public Health Office said in a press release. “It is critical that pregnant women in particular be treated promptly.”

Six of the 45 (13%) H1N1-associated deaths reported to the CDC between April 15 and June 16, 2009 occurred among otherwise-healthy pregnant women, whose infections progressed from primary viral pneumonia to acute respiratory distress and required mechanical ventilation. CDC officials attribute the deaths to delayed antiviral treatment.

“Testing is not available in many instances and, when available, results of novel H1N1 testing often takes several days,” the CDC stated in its interim antiviral guidance for pregnant women. “Clinicians should be aware of circulation of H1N1 in their area and not wait for test results to initiate influenza treatment in women who have symptoms consistent with influenza illness.”

Although the novel H1N1 has been widely susceptible to the neuraminidase inhibitors zanamivir (Relenza, GlaxoSmithKline) and oseltamivir (Tamiflu, Roche) thus far, the CDC has received five case reports of oseltamivir-resistant H1N1 influenza.

“That’s a very small number compared to the very large number of cases we’re seeing around the world,” Anne Schuchat, MD, director of the National Center for Immunization and Respiratory Diseases said during a press conference last week. However, she warned that influenza viruses have the potential to mutate quickly and may develop increasing resistance during relatively short periods of time.

Although it remains unclear just how severe illness caused by H1N1 will be in the fall, Schuchat said the high rates of influenza transmission observed this far into the summer may offer some clues about the disease’s prevalence in the upcoming months.

“Right now, 20 states are reporting widespread or regional influenza activity. It’s very unusual for that kind of illness to be occurring at this time of year. The novel H1N1 viruses are making up 98% of all subtyped influenza A viruses we have and we’re seeing them dominate here in the United States,” Schuchat said. She added that CDC officials are making their plans for the next two years based on a projection that 40% of the “workforce might be affected and not be able to work either because they were ill or because they needed to stay home to care for an ill family member.”

Regular communication with international field staff stationed in the southern hemisphere, where seasonal and novel H1N1 influenzas are currently co-circulating, suggest that H1N1 is capable of causing a range of illnesses from severe life-threatening diseases to milder illnesses where patients recover on their own, according to Schuchat.

Additionally, the CDC has recently published clinical characteristics of four patients who experienced influenza A H1N1-associated neurological complications including seizures, encephalitis and encephalopathy. All patients recovered fully and were discharged without neurologic sequelae, according to a report in Morbidity and Mortality Weekly Report. These complications have previously been reported with seasonal influenza viruses, but these are the first cases associated with the novel H1N1 virus.

Schuchat called vaccination “a very important part of the intervention tool-kit.” However, estimates from previous years suggest that less than 14% of pregnant women receive seasonal inactivated influenza vaccines, despite recommendations from the Advisory Committee on Immunization Practices and the American College of Physicians and Gynecologists.

Decisions regarding which populations clinicians will target with the new vaccine, if any, are expected to be made later today during a special ACIP H1N1 session. – by Nicole Blazek