THURSDAY, Dec. 4, 2014 (HealthDay News) -- This flu season looks like it could be worse than usual, due to an aggressive strain of influenza virus that might flout the protection provided by this year's vaccine, U.S. health officials warned Thursday.
A strain of influenza called H3N2 appears to be circulating most widely this season, and in the past death rates from H3N2 have been more than double that of other flu strains, according to officials from the U.S. Centers for Disease Control and Prevention.
"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," CDC Director Dr. Tom Frieden said during a news briefing.
To make matters more difficult, about half of the H3N2 viruses detected by CDC researchers so far appear to have mutated, and have genetically "drifted" away from the virus strain included in this year's flu vaccine.
"They're different enough that we're concerned that protection from vaccination against these 'drifted' H3N2 viruses may be lower than we usually see," Frieden said.
Five children already have died from the flu this season, said Dr. Joseph Bresee, chief of the Epidemiology and Prevention Branch in the CDC's National Center for Immunization and Respiratory Diseases.
Three of the five children had the H3 flu virus, although doctors don't know if they had the mutated form of the virus, Bresee said.
H3N2 viruses were predominant during the 2012-13, 2007-08 and 2003-04 flu seasons, the three seasons with the highest death rates in the past decade, the CDC officials noted.
Because vaccine protection is likely to be shakier than usual this season, CDC officials are urging doctors to use antiviral drugs as soon as possible for any suspected flu cases.
Drugs like Tamiflu and Relenza can't prevent flu, but will reduce the amount of time people are sick, Frieden said.
"Antivirals aren't a substitute for vaccination," Frieden said. "Vaccination prevents flu. But antivirals are an important second line of defense to treat the flu. And this year treatment with antiviral drugs is especially important, particularly for people who are at high risk for serious flu complications or for people who are very sick with flu."
The CDC is recommending that doctors not wait for the results of a flu test before starting patients on antiviral drugs, he said. Antivirals are most effective when given within two days of the onset of symptoms.
A CDC health advisory issued Wednesday urges doctors to aggressively use antiviral drugs in suspected flu patients who are:
- younger than 2 years old,
- 65 or older,
- suffering from chronic disease -- such as asthma, diabetes, heart or lung disease -- or have a suppressed immune system,
- morbidly obese,
- residents of nursing homes or chronic-care facilities.
CDC researchers first detected the "drifted" strains of H3N2 in March, when it was too late to include them in this year's vaccine, Frieden said. The World Health Organization issues its recommendations for the Northern Hemisphere vaccine in mid-February.
It takes about four months to make flu vaccine and ramp up production, Frieden said. This lag means that every year, immunologists have to take an educated guess as to which flu strains should be included in the vaccine.
Even though this year's vaccine does not directly protect against this particular H3N2 strain, Frieden still recommends that people get their annual flu shot.
The vaccine will protect against several active strains of flu, and could even provide some protection against mutated flu viruses, he said.
"If we have a severe season, getting a vaccine that provides even partial protection may be more important than ever," he added.
According to CDC surveillance data, seasonal flu activity has increased slightly but is still within regular parameters. Only Louisiana and Puerto Rico were experiencing high levels of influenza-like illness, as of Nov. 22.
For more on influenza, visit the U.S. Centers for Disease Control and Prevention.