WANTAGH, N.Y. (CBSNewYork) — More than one third of students in a single Long Island elementary school stayed home on Tuesday, with many of them suffering from flu systems.

As CBS2’s Carolyn Gusoff reported, health officials have been trying to gauge just how big the flu outbreak is.

With a daughter already home sick with the flu, Della Ng pulled her son out of school Tuesday as he came down with flu symptoms too.

“They get, like, a headache, and they have stuffy nose and a lot of fever,” Ng said.

Her children were among the 115 absent from Lee Road Elementary School in the Levittown School District. Of those, 32 had the flu, according to their parents.

Nassau County Health Commissioner Dr. Lawrence Eisenstein said it amounted to a flu cluster.

“We have a few confirmed cases, and our disease investigators right now are out evaluating why all the children are out. Certainly, not all of the children are absent because of the flu,” Eisenstein said.

Eisenstein said it could signal an early and active flu season.

The Health Department was to investigate each absence one by one to determine how many students actually had the flu and how many of had been vaccinated.

The Centers for Disease Control announced earlier this month that this year’s flu vaccine prevents fewer than half of flu cases, because the current strain of the virus mutated.

Ng’s children both received the shot.

“We don’t really know where they’re coming from,” she said. “It’s like everybody around here – they have it.”

The Levittown School District has asked parents to bring children with fever to the doctor, and assured parents that all touched surfaces have been thoroughly cleaned.

It was not clear how many parents have been keeping kids home to prevent infection, but doctors at Winthrop-University Hospital in Mineola said that measure may not do the trick.

“The reality is by the time a third of a school is affected, and the children are out, they’ve probably already been exposed — so that the horse may be out of the barn, so to speak,” said Dr. Leonard Krilov, chief of pediatric infectious diseases at the hospital.

The experts said the best protection is something most parents already know.

“I just tell the kids — wash hands; wash door knobs,” one woman said.

And Commissioner Eisenstein emphasized the flu vaccine should not be underestimated.

“It is the single best preventive measure that we have in medicine to help people avoid the flu,” he said.

He said if the vaccine does not prevent the virus, it will reduce the severity.

You can tell the difference between the common cold and the far more serious flu, in that the flu tends to start very suddenly and symptoms are more intense.

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