The earliest a vaccine could be ready is this fall, said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases.
During a press briefing Tuesday, Besser had said that the cases of infection found in the United States so far continued to be mild, but more severe cases were expected, and "as we move forward, I fully expect we will see deaths."
Many of the swine flu cases in the United States come from a New York City high school that had previously reported 18 cases of the infectious disease, Besser said.
Besser said the incubation period for the U.S. cases is two to seven days, which, he said, "is typical for what you see with an influenza virus."
The majority of new cases in New York continued to come at St. Francis Preparatory School in Queens. Some students who have come down with the infection had been to Mexico -- believed to be the source of the outbreak -- during a spring break trip to Cancun, the AP reported.
Texas has postponed all public high school sports and academic competitions at least until May 11 due to the outbreak.
As with the previously tested strains of the swine flu virus, new testing found that the pathogen remains susceptible to the two common antiviral drugs Tamiflu and Relenza, according to an April 28 dispatch from the CDC's Morbidity and Mortality Weekly Report.
The new flu strain is a combination of pig, bird and human viruses, prompting worries from health officials that humans may have no natural immunity to the virus.
Meanwhile, the AP reported Tuesday that the epidemic had crossed new borders, with the first cases confirmed in the Middle East and the Asia-Pacific region. Canada, Austria, New Zealand, Israel, Spain, Britain and Germany also have reported cases of swine flu
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