An aggressive type of influenza virus is already circulating, and some strains aren’t covered by this year’s flu vaccine.

The Centers for Disease Control and Prevention (CDC) is warning that this winter’s flu season may be a deadly one.

The agency said today that flu viruses in the H3N2 family are the most common so far this season. This type of flu virus often leads to more hospitalizations and deaths than other strains. In fact, in the three deadliest flu seasons of the past decade, H3N2 strains were the most common.

“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said CDC director Dr. Tom Frieden. “We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you’re sick.”

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Unfortunately, vaccines will only be of limited use against this season’s viruses. About half of the H3N2 viruses the CDC has analyzed are “drift” variants, or strains with significant differences from those targeted by this year’s flu vaccine. Flu vaccines contain weakened or dead bits of virus that train the body’s immune system to fight back.

Every year, a group of experts decides which flu viruses to target with that year’s vaccine. This year, the recommendations went out in mid-February. But the viruses continued to mutate, and the H3N2 drift variants appeared in late March.

The 2007 to 2008 flu season also saw widespread drift variants of the H3N2 virus. That year, the vaccine protected about 40 percent of those who got sick from H3N2. Sometimes, a drift virus will sicken a person who has been vaccinated, but the vaccine may weaken the symptoms.

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The CDC is still recommending that Americans get the flu vaccine. The agency also recommends that high-risk patients, including young children, adults older than 65, pregnant women, and people with chronic health conditions, such as asthma, diabetes, and heart or lung disease, start taking antiviral drugs at the first sign of illness. They should begin taking these medications even before doctors confirm that they have the flu.

Flu symptoms include fever, cough, sore throat, achiness, chills, and fatigue.

Common antiviral drugs used to treat the flu are oseltamivir (Tamiflu) and zanamivir (Relenza). These drugs provide the best results if people start taking them in the first 48 hours after flu symptoms appear.

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