WASHINGTON 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."
Japan reported four cases, and globally officials reported more than 4,200 people in 30 countries had been ill.
VERY GREAT UNDERESTIMATE
"We think this virus is in most of the United States," Schuchat said. "The individual numbers are likely to be a very great underestimate."
More Americans are seeing doctors for influenza-like illnesses at a time of year when such visits usually decline.
Schuchat said tests showed they do not all have the new H1N1 virus, however. Many have seasonal flu -- the H1N1 seasonal strain, the H3N2 seasonal strain and influenza B -- and even other infections.
Health experts have not openly criticized efforts by other countries to stop the virus from getting in -- most notably China and its territory of Hong Kong, which have quarantined travelers in contact with patients.
A spokeswoman in Hong Kong said on Saturday that a Mexican traveler confirmed as Hong Kong's first and only case of the new flu strain had been discharged from the hospital.
The unidentified man, who unwittingly caused the confinement of almost 300 guests and staff at a Hong Kong hotel where he had stayed, had been in hospital for a week.
China put seven people who had been exposed to three Japanese passengers diagnosed with the H1N1 flu in quarantine, the official Xinhua news agency quoted the government as saying.
The CDC said from the beginning it would be futile to try to stop the virus in the United States because it was only identified after person-to-person transmission had taken place for weeks before it was identified.
"Our indications are that it is still accelerating," Schuchat said. Mexican officials have said outbreaks there are on the wane, but Schuchat disagreed.
"In some parts of the country they may see a decline," she said, adding that fresh outbreaks were occurring elsewhere.
Mexican health ministry spokesman Carlos Olmos said the government was testing thousands of samples to confirm which patients with severe respiratory symptoms were actually infected with the flu.
He said more than 5,000 tests had been done on suspected cases and that 1,578 people were ill but were being treated.
After the virus was identified on April 23, Mexico banned public events and shut schools, bars, restaurants and many businesses to prevent people from gathering. Officials say disinfection of public spaces has helped control its spread.
Schools in the capital will reopen on Monday.
But the state government of Jalisco, home to Mexico's second-largest city Guadalajara, said schools, nightclubs and theaters will remain shut for another week after three suspected flu deaths.
Schuchat said it is not yet clear whether some measures taken have slowed the outbreak, but she said it was clear that early detection methods had alerted the world quickly.
She noted that the AIDS virus, which has now killed 25 million people globally and infects 33 million, spread for years before it was even identified.
"If we end up having a bad pandemic of influenza from this strain we would have had a real jump-start on things like vaccines," she said.
It could still turn into a dangerous form. "This particular virus had all of the hallmarks that we look for with a possible pandemic," she said. It was a new strain, capable of spreading easily and killing people.
Seasonal flu kills 250,000 to 500,000 people globally and infects up to a third of the population each year.
(Additional reporting by Daniel Trotta in Mexico City, Leah Eichler in Japan, James Pomfret, Nerilyn Tenorio and Jacqueline Wong in Hong Kong; Editing by Eric Walsh and Paul Simao)