- CDC says mutated flu virus means current flu shot is less protective
- Still, the CDC recommends people get vaccinated against the flu
- Flu activity across the country is currently low, CDC says
(CNN) -- Scientists are concerned about what they're seeing so far this flu season, the director of the Centers for Disease Control and Prevention said Thursday, a day after the agency advised doctors this year's flu vaccine is not as effective because the current strain of the virus has mutated.
Dr. Tom Frieden said researchers are worried that with this particular strain of the virus, "we could have a season that is more severe than most with more hospitalizations and more deaths."
The advisory sent Wednesday said 52% of the 85 influenza virus samples collected and analyzed from October 1 through November 22 were different than the virus strains included in this year's vaccine, indicating a mutation, or drift, of the strain.
The most common strain of the virus reported so far this season is influenza A (H3N2). In the past, this strain has been linked to higher rates of hospitalization and death, especially for those at high risk for complications, which is usually the very young, the elderly and those with chronic health conditions such as asthma or heart problems.
Doctor: You can't get flu from flu vaccine 
Finding relief this flu and cold season In February, a panel of experts decided on 3 strains of the virus to include in this year's vaccine, based on data about the common strains circulating around the globe throughout the year. Some years the vaccine is a better match to the circulating strains than others.
It's too late to create another version of the flu vaccine this year, Frieden said, because even with modern production technology, it typically takes about four months to produce the vaccine.
While this year's version is not as protective against a mutated strain, it can still decrease the severity of illness caused by the virus. It can also protect against other circulating strains of the virus, which is why the CDC still recommends getting the vaccine.
"The flu is bad, and you want to do anything you can to prevent getting it and to prevent giving it to other people," said Dr. Lisa Thebner, a pediatrician in New York City. "The vaccine isn't perfect, but it's the best protection we have for prevention."
When asked if people should be concerned, Thebner said "people should always be concerned about the flu."
In a news conference Thursday, Frieden emphasized that if you do start to see flu symptoms -- fever, sore throat, cough, body aches -- it's extremely important to begin taking antiviral medications as soon as possible. Tamiflu and Relenza have been considered most effective at reducing complications when given soon after symptoms begin.
However, he said, most doctors still do not treat people with the antiviral medications, noting that fewer than 1 in 6 people who are severely ill with flu get them. "It is very important that we do better," Frieden said.
Antiviral medications can reduce symptoms, shorten the length of time of the illness, and may keep people out of the hospital.
So far this year, five children in the United States have died from the flu, Frieden said. In past years, studies have shown 98% of children who do die from flu have not been vaccinated, but it was not clear if the five children Frieden cited were vaccinated.
During the 2012-2013 flu season 12,337 people were hospitalized with flu-related illness and 149 children died, according to CDC surveillance data. Ninety percent of those children were not vaccinated.
Overall, flu activity across the country is currently low, according to the latest flu outbreak data from the CDC.
