Influenza is an illness that kills many thousands of people worldwide every year and makes millions more seriously ill. Many of those victims are children. As Rose Hoban reports, new research shows vaccines prevent disease and death in children.

Flu vaccines are made in advance, based on projections of the strain of the coming season's flu virus. But often, the flu virus mutates, and that can reduce the effectiveness of a vaccine.

But now, new research finds that vaccinating children against flu prevents sickness and death even when the vaccine isn't a perfect match for the virus strain that eventually shows up.

"That can vary from season to season depending on how well the strains of influenza that are in the vaccine match the strains of influenza that actually end up circulating in the community," says epidemiologist Katherine Eisenberg from the University of Rochester.

Eisenberg and her colleagues examined information from a database of about 2,500 children who came into doctors' offices with flu-like symptoms over a two-year period.

"We saw who tested positive for flu and who tested negative. And then we went back and looked to see which of those children had been vaccinated," Eisenberg explains.

"And then we could compare the rates of vaccination among children who ended up getting the flu and children who came in with flu-like symptoms but turned out not to have flu."

The data also included information about children who were hospitalized with flu or cases of pneumonia that developed as a result of having had the flu.

"Nearly 60 percent of cases of influenza - based on children who showed up at a doctor's office or were hospitalized - may have been prevented by vaccine," Eisenberg says.

And she says some of the data came from a year when the viruses in the vaccine did not closely match the circulating flu virus.

"There is some evidence that even when it's not a perfect match, children receive some amount of protection, although not perfect protection from an unmatched vaccine," Eisenberg says. "And, as you get into older children, it may be that there is some amount of protection from vaccine in previous seasons."

Eisenberg says these findings just reinforce the importance of vaccinating children against flu every year.

Her research is published in the journal Pediatrics.