The Monday death of a 22-month-old Mexican toddler in Houston remains the only confirmed fatality outside Mexico from the disease, unofficially known as swine flu.
About 430 schools have been closed nationwide, 300 of them in Texas and 62 in Alabama. That represents less than half a percent of the nation’s 100,000 schools, according to the Department of Education.
Those two states have also canceled high school athletic events for the time being. Some colleges have called off graduation ceremonies, and the UC Regents changed a meeting scheduled next week in San Diego, saying they would meet instead by phone and at other campuses.
Harvard Medical School in Boston suspended classes until at least Wednesday after nine students in the dental school were identified as probable cases. One of the students contracted it from a friend who traveled to Mexico and apparently passed it on to classmates, officials said.
Dr. Anne Schuchat of the Centers for Disease Control and Prevention said a preliminary study indicated that in U.S. households with an infected victim, about a quarter of other household members also contracted the virus — a relatively modest proportion. In some pandemics, the rate has been as high as 35%.
Dr. Nancy Cox of CDC also said Friday that the new virus apparently lacks some of the genes that made the strain in the 1918 Spanish flu pandemic so deadly.
"We do not see the markers for virulence that were seen in the 1918 virus," she said.
However, she added, "there is still a great deal that we do not understand" about the H1N1 virus and its counterpart 91 years ago.
Mexican Health Secretary Jose Angel Cordova said Friday that the confirmed death toll from the H1N1 flu outbreak in his country was 16 and that the total number of cases nationwide had risen to 397. The country stopped reporting suspected infections when the number reached 2,500.
In a new report, however, the CDC said that it had confirmed H1N1 flu in 84 Mexican patients who died.
The figures have risen as samples from past flu patients are tested for the newly discovered virus.
Cordova also said public hospitals that cover about half the country had admitted 46 patients with severe flu symptoms Thursday, down from 212 on April 20.
"This is encouraging," he said.
CDC officials said that the mortality rate in Mexico may not be as high as originally feared because investigators initially missed many mild cases. As those cases are identified, the proportion of lethal cases has been declining.
As of Friday evening, the number of confirmed cases worldwide stood at 593 in 15 countries, with no new deaths reported.
Britain reported two new cases resulting from human-to-human transmission of the new virus, and Germany reported one, in a nurse caring for an H1N1 patient. Denmark and South Korea also reported their first cases.
Hong Kong reported a confirmed case, the first in Asia, in a Mexican national traveling through the city. Chinese health authorities have quarantined 200 guests and 100 staffers in the hotel where the traveler stayed and plan to give all of them the antiviral drug Tamiflu.
The paucity of new cases and the lack of sustained human-to-human transmission outside North America suggest that it is unlikely that the World Health Organization will increase its infectious-disease alert level any time soon.
Friday morning, Mexico began a five-day slowdown of its economy, urging a halt to all nonessential government and commercial operations in an effort to impede spread of the virus.
Carnival Cruise Lines said it was extending by a week a ban on stops at Mexican ports for all 16 of its ships, until at least May 11.
Continental Airlines said it was reducing the number of seats on flights to Mexico by about 50% because of reduced demand. The company said it would reduce the number of flights and in some cases use smaller planes.
United Airlines said it would cut its number of weekly flights to Mexico from 90 to 24 for the rest of the month. In June, the number will climb to 52, the carrier said. Delta Air Lines said it would also cut back on flights and use smaller planes.
In other airline news, a United flight from Germany to Washington was diverted to Boston after a woman on board reported flu-like symptoms. She was hospitalized in Boston, and the plane continued to its destination.
In Geneva, Dr. Marie-Paule Kieny of the WHO said laboratory tests using blood samples from people who received the seasonal flu vaccine showed it did not block the new virus.
"The current opinion and consensus is that there would be no protection," she said in a telephone news conference.
The United Nations agency has been in consultation with vaccine manufacturers, and some will probably shift production from seasonal vaccines to one targeting the H1N1 virus, she said.
Times staff writers Ken Ellingwood in Mexico City, Janet Stobart in London and Larry Gordon in Los Angeles contributed to this report.