New flu spreads to Australia, now common in U.S.

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - The new strain of H1N1 influenza popped up in Japan and Australia and has been confirmed in more than 2,000 Americans, but health officials said on Saturday the true number of cases was underestimated.

Although most cases appear to be mild, just as in seasonal flu the swine flu strain has killed, with 48 confirmed deaths in Mexico, two in the United States, one in Canada and one in Costa Rica.

It has moved into the southern hemisphere, where influenza season is just beginning, and could mix with circulating seasonal flu viruses or the H5N1 avian influenza virus to create new strains, health officials said.

"This is a very unusual circumstance," Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told a news briefing on Saturday.

"One of the big challenges with influenza viruses is the way that they change, the way they combine and their prevalence in a number of species," Schuchat added.

"This is why it is so important for countries to have a strong capacity to deal with influenza and also why it is very important to understand what happens at the interface between people and animals."

The CDC reported 2,254 confirmed U.S. cases of the virus with 104 people in the hospital, up from 1,639 cases previously. "Today there are almost 3,000 probable and confirmed cases here in the United States," Schuchat said.

"The good news is we are not seeing a rise above the epidemic threshold."