New York officials announced Thursday they were shutting down three schools in response to a swine flu outbreak and that one staff member had been hospitalized in serious condition.
The three schools, with a total of about 4,500 students, will close Friday and all next week in response to "an unusually high level of flu-like illnesses at those schools," Mayor Michael Bloomberg said.
The staffer, an assistant principal, was hospitalized, he added.
The outbreak came less than two weeks after New York health authorities pronounced the all clear at several other schools that briefly made the city the main location for swine flu cases outside of Mexico, where the disease apparently started.
Worldwide, more than 6,000 people have been confirmed contracting the swine flu virus, according to the latest figures Thursday. A total of 65 people have died, most of them in Mexico.
Bloomberg said swine flu had been identified in four students at one of the schools, all in New York's Queens neighborhood, "as well as in a staff member at the school who is critically ill."
"More than 50 students have been sent home from the school with flu-like symptoms" from that school, he said.
At a second school, 241 students reported absent Thursday, and at a third 29 students were documented with "influenza-like symptoms."
"Effective tomorrow we are going to close two intermediate schools and one elementary school in Queens for at least five full school days. So they will be closed all next week," Bloomberg said.
There were no further details regarding the condition of the critically ill assistant principal, but Bloomberg said there was "some indication, possibility of pre-existing conditions" adding to the effect of the swine flu itself.
"We're trying to ascertain whether those problems were exacerbated by the flu," Bloomberg said.
National health authorities earlier said that the number of confirmed cases of swine flu in the United States jumped Thursday from 3,352 to more than 4,000 in 47 states, plus the District of Columbia.
Three people have died of the new strain of swine influenza in the United States and health officials around the world have acted quickly to try to keep the spread of the virus under control.
Although symptoms and severity of the virus closely resemble ordinary flu, health experts say that the newness of H1N1 mean it could still pose a greater danger.
"While the symptoms of H1N1 flu seem to resemble those of seasonal flu the H1N1 virus appears to spread rapidly," Bloomberg said, also noting that May was outside the usual season for flu in the northern hemisphere.
Bloomberg acknowledged that the school closures would "impose difficulties on some families, but they will, we believe, only do so for a short time.
"We regret the inconvenience but we think these measures are absolutely necessary."
Just 10 days ago, city officials were celebrating the reopening of the school at the center of New York's first swine flu outbreak.
The school, also in Queens, had been closed for 10 days.
Most of the 73 confirmed cases in New York's initial outbreak were at that school.
Officials said that possibly hundreds were infected at the school, but that testing was abandoned because symptoms were so slight.
Only four US states have not seen the virus -- West Virginia, Mississippi, Wyoming and Alaska.
The northern state of Illinois tops the list with 620 confirmed cases, followed by Wisconsin (510), California (473) and Texas (439), according to the Centers for Disease Control earlier Thursday.