Two studies presented at a meeting of experts said that some swine flu victims remain contagious more than eight days after their symptoms have vanished, much longer than expected.
Currently, US health authorities recommend that people who contract the A(H1N1) virus wait 24 hours after the fever has subsided before returning to their normal activities to avoid any risk of spreading the disease.
However the two studies, one conducted in Canada and the other in Singapore, concluded that between 19 and 30 percent of people infected with swine flu may remain contagious for eight days or more after their fever has vanished.
However none of the patients showed any trace of the virus 10 days after fever broke, De Serres said.
"This study shows you're not contagious for a day or two. You're probably contagious for about a week" after the fever has abated, De Serres told reporters at the American Society for Microbiology's 49th annual conference on antimicrobial agents and chemotherapy meeting this week.
A second study conducted by David Lye of the Tan Tock Seng Hospital in Singapore found that 20 to 30 percent of a group of 70 swine flu patients still carried the live virus, and were potentially infectious, eight days after fever ended, and a few still remained contagious up to 16 days later.
However the infectious period was shorter for patients treated with antiviral medicine.
De Serres said that the Canadian study did not look at the density of the viral load at the eight-day mark, but that tests were continuing. This information is key in estimating the true risk of contagion.
"The current recommendation suggests that fever subsides in three to four days, and people go work by fifth day," said Daniel Jernigan, deputy director of the influenza division at the US Centers for Disease Control and Prevention.
"Some people will continue to shed live virus after their fever stops, we know that," he said, adding that current recommendations "are intended to decrease infection, 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 said.
While noting that both studies were carried out on small sample groups, Frank Lowy, a virologist at Columbia University in New York, told reporters that if confirmed in new studies, the results could be significant in efforts to contain the swine flu spread.