Early data suggest the current 2014-2015 flu season could be severe. The Centers for Disease Control and Prevention urges immediate vaccination for anyone still unvaccinated this season and recommends prompt treatment with antiviral drugs for people at high risk of complications who develop flu.

So far this year, seasonal influenza A H3N2 viruses have been most common. There often are more severe flu illnesses, hospitalizations, and deaths during seasons when these viruses predominate. For example, H3N2 viruses were predominant during the 2012-2013, 2007-2008, and 2003-2004 seasons, the three seasons with the highest mortality levels in the past decade. All were characterized as “moderately severe.”

Increasing the risk of a severe flu season is the finding that roughly half of the H3N2 viruses analyzed are drift variants: viruses with antigenic or genetic changes that make them different from that season’s vaccine virus. This means the vaccine’s ability to protect against those viruses may be reduced, although vaccinated people may have a milder illness if they do become infected. During the 2007-2008 flu season, the predominant H3N2 virus was a drift variant yet the vaccine had an overall efficacy of 37 percent and 42 percent against H3N2 viruses.

“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said CDC director Tom Frieden, M.D., M.P.H. “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, to reduce flu spread.”

Depending on the formulation, 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.

“While the vaccine’s ability to protect against drifted H3N2 viruses this season may be reduced, we are still strongly recommending vaccination,” said Joseph Bresee, M.D., Chief of the Influenza Epidemiology and Prevention Branch at CDC. “Vaccination has been found to provide some protection against drifted viruses in past seasons. Also, vaccination will offer protection against other flu viruses that may become more common later in the season.”

Influenza viruses are constantly changing. The drifted H3N2 viruses were first detected in late March 2014, after World Health Organization (WHO) recommendations for the 2014-2015 Northern Hemisphere vaccine had been made in mid-February. At that time, a very small number of these viruses had been found among the thousands of specimens that had been collected and tested.

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. “We are just at the beginning of the season. It’s not too late to get your vaccine,” Dr. Frieden says.

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 soon after symptoms develop as possible, without waiting for lab tests to confirm flu infection.

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.

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.