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Swine Flu Infects Seven; Genetic Make-Up Has Scientists Stymied

A swine flu strain that has infected seven people in the United States since late March is an unusual hybrid that carries genetic material from four different sources, officials at the U.S. Centers for Disease Control and Prevention said this afternoon. The agency says the situation is no reason for major concern yet, since the spread of the virus appears limited, and unlike the avian H5N1 flu, it does not appear to cause a very severe disease. But the virus can apparently be transmitted between humans, raising concerns that an epidemic—or even a pandemic—is possible, and officials have asked doctors and labs around the country to be on the lookout.

On Tuesday, CDC officials reported that two children in California had become infected with a swine influenza strain. Since then, five more cases of the swine flu have been confirmed through lab analysis, Anne Schuchat, interim deputy director of CDC's Science and Public Health Program, told reporters during a teleconference this afternoon. Three of those occurred in California, and two near San Antonio, Texas. None of the patients has been in contact with pigs. It's unclear whether there's a link to recent reports about a severe respiratory illness in Mexico.

All seven patients have recovered, and although one of them had to be hospitalized, "so far, it's not looking like a very severe influenza," Schuchat said. Lab tests have shown that the virus is resistant to two old influenza drugs called rimantadine and amantadine, but is susceptible to two newer drugs, oseltamivir and zanamivir, which have a different mode of action.

The enigmatic virus is an H1N1 influenza strain that normally infects swine.

Researchers occasionally find human infections with swine flu viruses; a human outbreak of the virus in 1976 triggered a massive vaccination campaign that many now say was an overreaction, and that was cancelled after the vaccine caused severe side effects in an unusual number of people.

Preliminary genetic analysis has shown that the virus appears to be an unusual hybrid that has genetic material from four different sources, Nancy Cox, chief of CDC's Influenza Division, said at the teleconference: avian and swine viruses from North America, a swine flu strain usually seen in Asia, and a human influenza strain. It's unclear how the virus picked up the odd combination. CDC is preparing a so-called seed strain that could be used to make a vaccine, a process that can take several weeks and that is "standard operating procedure" whenever a new influenza strain is going around, Cox said.

CDC expects to confirm many more cases in the days ahead. But at today's press conference, Schuchat cautioned against panic, stressing that the seven cases so far may have simply been identified as a result of improved surveillance. (The two cases that started the investigation were found thanks to the Border Infectious Disease Survillance, a network of clinics that conducts surveillance along he U.S.-Mexican border, and a naval lab in California with advanced diagnostic capabilities.) "We don't think it's time for major concern around the country," Schuchat said.

CDC will provide updates on the situation every day around 3 p.m. in the weeks ahead.