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.
This includes children under age 2, adults 65 and older, people with chronic pulmonary and cardiovascular problems and those with neurologic and immunosuppressant disorders.
It also includes pregnant and postpartum women, people under age 19 who are on long-term aspirin therapy, American Indians, the morbidly obese and nursing home and chronic-care facility residents.
Antivirals are most effective when taken within 48 hours of the onset of flu symptoms. But fewer than one in six flu patients actually get the drugs, Frieden said.
“It’s very important that we do better. We need to get the message out that treating early with these drugs can make a difference between having a milder illness or a very severe illness,” Frieden said.
Many doctors don’t prescribe antivirals because they mistakenly believe that patients must first be tested for the virus. Others just aren’t familiar with using antivirals for flu treatment because they’re most effective in treating bacterial infections, Frieden said.
The drifted viruses were first detected in March. Frieden said their emergence could have resulted from their attempts to escape the immunity being developed by widespread vaccination. The drifted viruses became prominent in September when it was too late to include them in this year’s vaccine formula. It takes four months to make the vaccine.
“Up until now, there’s been so little activity it was hard to say that this virus was going to be of public health importance or not,” said Dr. Joseph Bresee of CDC’s National Center for Immunization and Respiratory Diseases. “I think now, with the increasing cases of flu, and (the fact) that this virus is maintaining a presence and maintaining a prevalence in the United States, we felt it necessary to come out and talk about it a little bit.”
Currently, four different strains of flu are circulating in the U.S., and “only time will tell which of them, if any, will predominate for the following weeks and months of this year’s flu season,” Frieden said.
The flu vaccine provides protection against several of the virus strains and may provide partial protection against the drifted strains, Frieden said.
About 146 million doses of the vaccine have been distributed in the U.S. and there is no shortage.