MEXICO CITY — Alarmed Mexican officials, scrambling to control a swine flu outbreak that has killed as many as 68 people and infected possibly 1,000 more in recent weeks, canceled more public events Saturday in and around the capital and said they were considering keeping schools for millions of students here closed into next week.
Officials also announced that two soccer matches scheduled for Sunday would be played without spectators, and hundreds of cultural events have been canceled.
The Associated Press reported Saturday that 24 new suspected cases of the flu were reported.
Mexico’s health minister, José Ángel Córdova, has said the country is dealing with a new flu virus that constitutes a respiratory epidemic that is so far controllable. He said the virus had mutated from pigs and had at some point been transmitted to humans.
The new strain contains gene sequences from North American and Eurasian swine flus, North American bird flu and North American human flu, said the Centers for Disease Control and Prevention. A similar virus has been found in the American Southwest, where officials have reported eight nonfatal cases.
Most of Mexico’s dead were young, healthy adults, and none were over 60 or under 3 years old, the World Health Organization said. That alarms health officials because seasonal flus cause most of their deaths among infants and bedridden elderly people, but pandemic flus — like the 1918 Spanish flu, and the 1957 and 1968 pandemics — often strike young, healthy people the hardest.
Mexican officials have been urging people to avoid large gatherings and to refrain from shaking hands or greeting women with a kiss on the right cheek, as is common in Mexico.
On Friday, Mexico City closed museums and other cultural venues, and advised people not to attend movies or public events. Seven million students, from kindergartners to college students, were kept from classes in Mexico City and the neighboring State of Mexico on Friday, in what news organizations called the first citywide closing of schools since a powerful earthquake in 1985.
Because of the situation, the World Health Organization planned to consider raising the world pandemic flu alert to 4 from 3. Such a high level of alert — meaning that sustained human-to-human transmission of a new virus has been detected — has not been reached in recent years, even with the H5N1 avian flu circulating in Asia and Egypt, and would “really raise the hackles of everyone around the world,” said Dr. Robert G. Webster, a flu virus expert at St. Jude Children’s Research Hospital in Memphis.
Mexico’s flu season is usually over by now, but health officials have noticed a significant spike in flu cases since mid-March. The W.H.O. said there had been 800 cases in Mexico in recent weeks, 60 of them fatal, of a flulike illness that appeared to be more serious than the regular seasonal flu. Mr. Córdova said Friday that there were 1,004 possible cases.
Still, only a small number have been confirmed as cases of the new H1N1 swine flu, according to Gregory Hartl, a W.H.O. spokesman. Mexican authorities confirmed 16 deaths from swine flu and said 45 others were under investigation, most of them in the Mexico City area. The C.D.C. said that eight nonfatal cases had been confirmed in the United States, and that it had sent teams to California and Texas to investigate.
“We are worried,” said Dr. Richard Besser, the acting head of the C.D.C. “We don’t know if this will lead to the next pandemic, but we will be monitoring it and taking it seriously.”
There is no point in trying to use containment measures in the United States, he said, because the swine flu virus has already appeared from San Antonio to San Diego, without any obvious connections among cases. Containment measures usually work only when a disease is confined to a small area, he said.
The C.D.C. refrained from warning people not to visit Mexico. Even so, the outbreak comes at an awful time for tourism officials, who have been struggling to counter the perception that violence has made Mexico unsafe for travelers. The outbreak was also causing alarm among Mexicans, many of whom rushed to buy masks or get checkups.
“I hope it’s not something grave,” said Claudia Cruz, who took her 11-year-old son, Efrain, to a clinic on Friday after hearing the government warnings.
Health officials urged anyone with a fever, a cough, a sore throat, shortness of breath or muscle and joint pain to seek medical attention.
When a new virus emerges, it can sweep through the population, said Dr. Anne Moscona, a flu specialist at Cornell University’s medical school. The Spanish flu is believed to have infected at least 25 percent of the United States population, but killed less than 3 percent of those infected.
The leading theory on why so many young, healthy people die in pandemics is the “cytokine storm,” in which vigorous immune systems pour out antibodies to attack the new virus. That can inflame lung cells until they leak fluid, which can overwhelm the lungs, Dr. Moscona said.
But older people who have had the flu repeatedly in their lives may have some antibodies that provide cross-protection to the new strain, she said. And immune responses among the aged are not as vigorous.
Despite the alarm in recent years over the H5N1 avian flu, which is still circulating in China, Indonesia, Egypt and elsewhere, some flu experts argued that it would never cause a pandemic, because no H5 strain ever had. All previous pandemics have been caused by H1s, H2s or H3s.
Among the swine flu cases in the United States, none had had any contact with pigs; cases involving a father and daughter and two 16-year-old schoolmates convinced the authorities that the virus was being transmitted from person to person.
In Canada, hit by the SARS epidemic in 2003, health officials urged those who had recently traveled to Mexico and become ill to seek treatment immediately.
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