The latest reports on H1N1 vaccine are that just one dose may be enough to fight infection. But clinical trials starting this week among pregnant women will still test the effectiveness of two shots.

Duke Medical Center is one of six medical centers in the country participating in the trial. Patients who are seeing Duke doctors during their pregnancy are eligible to participate if they are in good health, are between 18 and 39 years old and are in their second or third trimester of pregnancy.

Duke Obstetrician Dr. Geeta Swamy said the trial will examine how immune system changes that happen naturally during pregnancy are affected by vaccination. Blood samples from participants will measure the immune response and may help with a final determination on how many doses are necessary.

"We know that influenza vaccination in general is recommended for pregnant women because of the high risk that is associated with influenza during pregnancy," she said. "But what we don't know a lot about is how the immune system actually responds or functions during pregnancy with regards to vaccination."

Swamy said patients with H1N1 appear to get sick faster than with the regular flu. The danger for pregnant women is the risk of complications that also can happen more quickly.

"Often during seasonal influenza, we'll see that people have influenza-like symptoms and go on later to have other complications, like bacterial pneumonia and so forth," she said. "With H1N1, we're seeing a much faster onset to more complicated cases."

Swamy said she is encouraging her patients to avoid close contact with people who are sick, wash their hands frequently to avoid infection, and watch for symptoms, including cough, cold and/or fever over 100 degrees.

"It's mostly people who have children who have a thermometer at home, so many of us often don't take our own temperature," she said. "But it's important to take your own temperature and know what it is without taking any fever-reducing medication."

Pregnant women who have flu-like symptoms should contact their doctor immediately.

"As obstetricians, we are accustomed to being on call 24- hours-a-day," said Swamy.

The initial trial will include just 20 patients, but Swamy anticipates there will be several more trials at Duke over the next few months.