In another large U.S. outbreak, New York City announced Friday it was closing three more schools where students have been sickened with swine flu symptoms. That brings the number of shuttered schools to six.
A city official says the new schools are in Queens and Brooklyn.
Officials said Friday the virus is spreading faster and more erratically than seasonal flu does, but the symptoms have generally been mild. Hundreds of schoolchildren were sent home sick this week and an assistant principal remains in critical condition with a confirmed case.
Health Commissioner Dr. Thomas Frieden said the large clusters of cases are "a little surprising," and officials don't know why the virus is spreading more rapidly than traditional influenza. Frieden was appointed by President Barack Obama to head the Centers for Disease Control and Prevention on Friday.
The CDC has said repeatedly that there would be more deaths and new cases, and that the strain was still on the upswing in the U.S. Texas on Friday reported the nation's fifth swine flu death. The H1N1 virus has been confirmed in more than 4,700 people in United States and nearly 8,000 worldwide.
The agency also announced Friday that it plans to downgrade its warnings about travel to Mexico to just a precaution for those at high risk of flu complications because of underlying health conditions.
Hundreds home from school
The schools were closed Thursday after four students and the assistant principal at the Susan B. Anthony middle school in Queens were confirmed to have swine flu. Mayor Michael Bloomberg said there were no immediate plans to close more schools.
The mayor said Thursday that the 55-year-old assistant principal may have had pre-existing health problems — but on Friday relatives denied that.
It isn't unusual for flu cases to ebb and surge as the virus moves through a population during flu season. The virus tends to disappear as the weather gets warmer and more humid.
Dr. Isaac Weisfuse, a deputy commissioner of the health department, said investigators are trying to learn more about why the disease has spread erratically.
Schools are a good incubator for illness in general, he said, because space is tight and youngsters often don't practice the best hygiene.
Frieden said Friday that officials look at a number of factors when deciding whether to close a school, including how many kids are out sick. "It has to be a school-by-school and day-by-day assessment," he said.
New York City's first outbreak occurred when hundreds of teenagers at a Roman Catholic high school in Queens began falling ill following the return of several students from vacations in Mexico, where the outbreak began.
An estimated 1,000 students, their relatives and staff at the St. Francis Preparatory School fell ill in a matter of days.
Additional sporadic cases continued to be diagnosed, but the symptoms were nearly all mild. The sick children recovered in short order and St. Francis reopened after being closed for a week. The middle school with the confirmed cases is two miles from St. Francis.