More than 2,000 people in 23 countries worldwide now have confirmed cases of the new strain of the H1N1 flu, the World Health Organization said on Thursday.
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A bus stop in Mexico City on Wednesday as some people began returning to work.
Over half of the 2,099 laboratory confirmed cases are in Mexico, the apparent epicenter of the virus, where 44 people are confirmed to have died of the disease, popularly referred to as swine flu. Thirty percent of the W.H.O. confirmed cases are in the United States, where authorities say the virus is now widespread, and two deaths have been reported.
Some state health authorities have said they no are longer testing every possible case of the virus, which has proved less deadly than initially feared and has mostly resulted in mild infections. In New York state, for example, authorities said Wednesday they will test only enough to follow overall trends in the virus, as they do with seasonal flu. The national tally of confirmed cases, which rose on Thursday to 896 cases in 41 states, is therefore only a general picture of the virus’s spread.
“We expect to see large increases in these numbers in the coming days,” said Dr. Richard E. Besser, the acting director of the Centers for Disease Control and Prevention, on Thursday, both because of new cases, and because there are large backlogs of laboratory tests in many states. “The virus is continuing to spread both domestically and around the world,” he said.
In Mexico, where the outbreak appears to be ebbing, university and high school students returned to school on Thursday. Restaurants reopened, but with strict health guidelines, including that all restaurant staff wear face masks. Authorities continue to recommend face masks for those traveling on public transportation in Mexico City.
Health officials also continued to warn that the outbreak’s course remains uncertain, and that the virus could easily change and become much more ferocious.
Dr. Keiji Fukuda, the W.H.O.’s interim assistant director-general, urged governments on Thursday to stay alert for a possible wider pandemic. Given the history of previous pandemics, he said, “it would be a reasonable expectation that one-third of the world would get infected” over the next year or two if this flu became a pandemic. In a world of six billion people, “that’s a lot of people,” he said.
The total number of cases in Europe rose to 147, the European Centre for Disease Control and Prevention said, with Spain and Britain reporting the most cases. There is still no sustained person-to-person transmission of the virus in any country in Europe, however, authorities said.
In Shanghai on Thursday, travelers from Mexico began to be released after spending a week in quarantine, while some 350 guests of a quarantined hotel in Hong Kong were due to be released Thursday night.
Serious cases of the virus have particularly struck young people for reasons that are still not fully understood by international health authorities. Although few Americans have been hospitalized with serious swine flu complications, their median age is 15, Dr. Besser said. “This is something that’s raising concern,” he said Wednesday. “It’s something we’re keeping an eye on.”
About five percent of the country’s 896 confirmed cases were hospitalized as of Thursday morning, which is higher than would be the case for seasonal flu. But Dr. Besser pointed out that states are concentrating on testing only the most serious cases for the novel virus, so the actual percentage of serious cases among all sufferers could be lower.
He also cautioned against “overinterpretation” from such a small number of hospitalizations. The fact that many early cases were students who had spent spring break in Mexico could make the flu’s spread more common in teenagers, he said. And some scientists have speculated that anyone born before 1957 has been exposed to a similar H1N1 seasonal flu and may have some immunity.
Dr. Besser also said that holding “swine flu” parties or otherwise trying to deliberately get infected with the virus on the theory that it would provide immunity against catching it if it returns in the fall “is a big mistake.”
“How an individual person will be impacted by the infection is something we do not know,” he said. “The C.D.C. does not recommend that people follow that course.”
In Mexico, where the epidemic is believed to have started in March, hospitalizations and deaths among previously healthy young people alerted authorities that something other than seasonal flu was circulating. Although seasonal flu causes 250,000 to 500,000 deaths annually around the world, the vast majority are of the elderly, of infants and of people with other health problems.
Some of the more than 300 new confirmed cases reported by the C.D.C. on Thursday come from a backlog of cases; others are new cases tested as recently as Tuesday, a spokesman said.
Illinois is now the state with the most cases, 204, surpassing New York, with 98 cases, possibly because Illinois is testing more. In Albany, the state health commissioner, Dr. Richard F. Daines, said New York had cut back on testing and would suspend daily news briefings and treat the virus much like the seasonal flus that sweep the nation each year.
“We need no longer conduct surveillance to determine the existence of the virus,” he said. Dr. Besser said, as he has before, that he expected the flu to spread to all 50 states and to cause more serious cases.
The question of which cases to test has been troubling to some doctors, said Dr. Anne Moscona, a flu specialist at Weill Medical College of Cornell University.
“The most recent guidance over the past week has been that unless someone is seriously ill, has a high likelihood of having been exposed, or is at serious risk of complications, do not test them and do not treat them with anti-virals,” she said. “I’m anticipating that guidance may be relaxed a little bit now that we know that what we are looking at looks more like the seasonal flu.”
The C.D.C. has now issued flu testing kits to all states and Puerto Rico, which it says should result in a clearer picture of the virus’s spread, the organization reported on its Web site.
Joseph Bresee, chief of epidemiology and prevention for influenza at the C.D.C., said that federal authorities recommend that the focus of testing be on severe cases and cases at high risk of becoming severe but that “routine” testing is recommended to capture the milder illnesses.
“The case counts are important for following trends over time, but are not intended to be interpreted as the total number of cases,” he said in an email message. “We have a variety of methods and systems that together will provide the whole picture of the impact of the outbreak.”