WASHINGTON — This season’s flu vaccine may not be as effective against new and possibly more virulent strains of the virus that could become the nation’s dominant form of influenza in the young flu season.

The emergence of a mutated flu virus has caused the Centers for Disease Control and Prevention to issue a national health advisory urging caregivers to prescribe the antiviral medications Tamiflu and Relenza to suspected flu patients, even before they’ve been confirmed to have the virus.

Although flu activity has been relatively mild in the U.S. thus far, Influenza A viruses, known as H3N2, have been seen in most states.

“We know that in seasons when H3 viruses predominate, we tend to have seasons that are worse flu years with more hospitalizations from flu and more deaths from influenza. . The rate of hospitalization and death can be twice as high or more,” said CDC Director Tom Frieden in a conference call with reporters.

So far this season, five children have died from the flu, and outbreaks have occurred in schools and nursing homes, Frieden said.

Testing has found that 52 percent of more than 1,100 H3N2 virus samples were found to be antigenetically different, or “drifted,” from the H3N2 virus used in this year’s flu vaccine, Frieden said.

“They’re different enough that we’re concerned that protection from vaccination against the drifted H3N2 viruses may be lower than we usually see,” Frieden said. “These changes can signal that the immune response provided by vaccination won’t protect as well for these viruses.”

Despite the prospect of lower protection rates, Frieden said vaccination is still the best protection against the flu “and may have some effectiveness against the drifted strain.”

To fight complications from the flu, Frieden said antiviral medications, which shorten the duration and severity of flu symptoms, are an “important second line of defense,” particularly for people who are at high risk of serious flu complications.