Swine Flu: How Long Are You Infectious?

Some People Shed H1N1 Virus More Than a Week After Symptoms Strike

Medically Reviewed by Louise Chang, MD on September 15, 2009

Sept. 15, 2009 (San Francisco) -- Some swine flu patients are still infected with H1N1 virus that they can transmit to other people eight to 10 days after their symptoms strike, researchers say.

The finding suggests that the CDC’s recommendation for people with flu-like illness to avoid others until at least 24 hours after they are free of fever may not go far enough.

“Three days after your fever subsides, you’re probably still contagious with H1N1 [swine] flu,” says Gaston De Serres, MD, PhD, a medical epidemiologist at the National Institute of Public Health in Quebec.

De Serres tells WebMD that it’s not yet known exactly when it is safe to go back to school or work. “But a couple of days [after symptoms strike] is likely to be insufficient. You’re probably contagious for about a week.”

New Studies on Swine Flu Spread

De Serres led one of two teams of researchers that separately reported this week that as many as one in five patients continue shedding H1N1 virus eight days after symptoms start.

Both studies were presented here at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy.

For the first study, De Serres and colleagues swabbed the noses and throats of 43 patients with confirmed H1N1 flu as well as those of their sick family members.

On the eighth day after symptoms struck, 8% to 19% were still shedding live H1N1 virus.

The second study, led by David C. Lye, MD, and colleagues at Tan Tock Seng Hospital in Singapore, involved 70 patients. It showed that 80% were still shedding virus after five days of illness, 40% at seven days, and 10% at 10 days.

People who were treated with Tamiflu shed virus for an average of three fewer days than those not given the anti-flu drug, “but still, some were showing signs of viral shedding after a week,” Lye tells WebMD.

Tamiflu or another anti-flu drug, Relenza, is recommended for anyone hospitalized with flu-like symptoms or at high risk of complications. The drugs, which should optimally be started within two days of symptom onset, shorten illness by about one day.

Neither study used methods sensitive enough to determine whether people were shedding enough virus to spread infection.

CDC Perspective

More than 1 million Americans have been infected with the H1N1 virus and nearly 600 people have died from the swine flu, according to the CDC.

The CDC guidelines state that people with flu-like symptoms should stay home and not go back to school or work until at least 24 hours after they are free of a fever of 100 degrees Fahrenheit or more, or signs of a fever, without using fever-reducing medicines.

People are generally feverish for one to three days after becoming ill. And “symptoms such as cough can last for two weeks or more,” De Serres says.

Daniel Jernigan, MD, deputy director of CDC's flu division, says, "Some people will continue to shed live virus after their fever stops; we know that. The policies are intended to decrease, but not completely eliminate, infection."

"If we had a virus with a very high attack rate or death rate, we might have a very different policy,” he tells WebMD.

Frank Lowy, MD, an infectious disease expert at Columbia University College of Physicians & Surgeons in New York, says that if confirmed, the new findings may help to explain why the H1N1 pandemic has spread so quickly. Lowy was not involved with the work.

Show Sources


49th Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, Sept. 12-15, 2009.

Gaston De Serres, MD, PhD, medical epidemiologist, National Institute of Public Health, Quebec.

David C. Lye, MD, Tan Tock Seng Hospital, Singapore.

Daniel Jernigan, MD, deputy director, flu division, CDC.

Frank Lowy, MD, Columbia University College of Physicians & Surgeons, New York.

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