Health officials have been telling people to avoid contact with others for a day after their fever goes away. The new research suggests they may need to be careful for longer, especially at home where the risk of spreading the germ is highest.

"This study shows you're not contagious for a day or two. You're probably contagious for about a week," said Dr. Gaston De Serres, a scientist at the Institute of Public Health in Quebec, Canada.

He presented one of the studies today at a microbiology conference.

It is unclear whether the new research will prompt the U.S. Centers for Disease Control and Prevention to rethink its advice. Long breaks from school and work do not seem worth it for a virus that now seems to cause mostly mild illness, said the CDC's flu chief, Nancy Cox. Swine flu is spreading so widely now that confining the sick does less good, she said.

"We tried to have our guidance balance out all of these factors," she said. "It's just virtually impossible not to have virus introduced into settings such as schools and universities."