The Centers for Disease Control (CDC) warns that this year's flu season could be a bad one. Drawing on data showing that this year’s most commonly reported flu strains are those responsible for the most deadly flu seasons of the decade. As a result of that data, the CDC urges everyone to get a flu vaccine this year.

The CDC is also recommending that those at high risk of complications from the flu get prompt treatment with Tamiflu or Relenza if they catch the flu. Those at high risk from influenza include children younger than 5 years (especially those younger than 2 years); adults 65 years and older; pregnant women; and people with certain chronic health conditions such as asthma, diabetes, heart or lung disease, and kidney disease.

The CDC said that the flu viruses most common this year are A H3N2 strains, which often cause more severe illnesses, hospitalizations and deaths than some other strains. In addition, about half of the viruses seen during the flu season to date have genetically mutated from the vaccine virus, significantly reducing the vaccine 's effectiveness.

Depending on the formulation, the agency said, flu vaccines protect against three or four different flu viruses. Even during a season when the vaccine is only partially protective against one flu virus, it can protect against the others, that may become prevalent later in the season.

Influenza viruses are constantly changing, the CDC said. The mutated, or “drifted” H3N2 viruses were first detected in very small numbers in late March 2014, after World Health Organization recommendations for the 2014-2015 vaccine had been made.

A committee of experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered in time for the upcoming flu season. There is always the possibility that viruses will drift during that time.

Influenza activity is currently low in the United States as a whole, but is increasing in parts of the country and the CDC warns that it is still early in the flu season. They say there is still time to get a flu shot.

The CDC says that influenza antiviral drugs - Tamiflu (oseltamivir) and Relenza (zanamivir) can reduce severe complications such as hospitalization and potentially death for people who are at high risk of serious flu complications or are very sick. Treatment of high risk patients should begin as quickly as possible after symptoms develop, without waiting for lab tests to confirm flu infection.

CDC recommends that people at high risk check with their doctor or other health care professional promptly if they get flu symptoms. Studies show that flu antiviral drugs work best for treatment when they are started in the first 48 hours after symptoms appear. Flu symptoms can include fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and fatigue.