This year's flu season could be especially severe, US health authorities warned Thursday, adding that genetic mutations mean the vaccine won't be as effective against some of the most prevalent flu strains.

So far this season, most flu patients in the United States have contracted seasonal influenza A H3N2 viruses.

This group of viruses was also the most common in the three deadliest flu seasons over the past decade, said officials at the Centers for Disease Control and Prevention.

Perhaps more worrying, the officials said, was that analysts have found genetic mutations in around half the H3N2 viruses analyzed this season.

The mutations mean the viruses no longer resemble the one used to develop this season's flu shot -- which means even vaccinated people will have limited protection.

However, they noted it was still important to get flu shots -- emphasizing it could still help lessen the severity of symptoms, even if it doesn't block the virus entirely.

"We are still strongly recommending vaccination," said Joseph Bresee, chief of the Influenza Epidemiology and Prevention Branch at CDC.

"Vaccination has been found to provide some protection against drifted viruses in past seasons," he added.

"Also, vaccination will offer protection against other flu viruses that may become more common later in the season."

In the 2007-2008 flu season -- seasons typically peak between December and February -- the predominant flu variation was a mutated H3N2 virus, the CDC said, but the vaccine still had an overall efficacy of 37 percent and 42 percent against H3N2 viruses.

The vaccine must be created months in advance in order to be ready for the upcoming flu season, and there is always the possibility the viruses will "drift" genetically in the meantime.

Currently, flu cases are still quite low in the United States.

"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, reiterating calls for everyone to get vaccinated.

The CDC stressed that those most at risk from the flu -- children under age five, pregnant women, and people over 65 -- should be especially vigilant and seek treatment as soon as symptoms appear.