Flu cases have surged by more than 119 percent in recent weeks throughout New York, where a majority of counties -- including Nassau and Suffolk -- are reporting widespread flu activity.
"The season can continue into spring, but we are really starting to see an increase in intensity," said Dr. Gary Leonardi, chief of virology at Nassau University Medical Center, who monitors flu activity for the state Health Department and the U.S. Centers for Disease Control and Prevention.
As a direct consequence of a rising number of flu infections, hospitalizations statewide have also increased, according to state Health Department data. An estimated 287 people have been confined to hospitals throughout New York in recent weeks, an increase of 126 percent compared with earlier in the flu season, which began in October. Pneumonia is the most common flu-related complication, state health officials said.
New York is one of more than 20 states reporting a surge in flu activity in recent weeks, according to the CDC.
Leonardi said even though flu activity is rising, there is no evidence of an epidemic.
Yet, that doesn't mean communities are in the clear.
Epidemics emerge as a result of numerous factors, such as the virulence of the dominant strain and the number of people who have not been vaccinated. There also can be a switch from one dominant strain to another.
The most prevalent flu virus now circulating is A/2009 H1N1, a sibling strain to the one that stoked fears of a world-wide pandemic five years ago.
People most vulnerable, according to state and national data, range from 5 to 49 years old, with a majority of cases reported among young adults, 18 to 40.
"The elderly are not getting infected as much," Leonardi said, "and that may cut down on mortality this year."
Normally, people over 65 bear the brunt of flu complications.
Leonardi noted the A/2009 H1N1 component in the vaccine is a strong match against the circulating strain.
As with all flu viruses on the move, H1N1 has undergone distinct evolutionary changes since its abrupt appearance in 2009, infectious-disease experts say.
Dr. Susan Donelan, medical director for health care epidemiology at Stony Brook University Hospital, said circulating flu viruses make changes to their outer proteins all the time, mutations that allow them to stay several steps ahead of the human immune system. Constant mutations are what keep flu viruses infectious.
Donelan said these types of mutations are formally called antigenic drift.
"This can occur multiple times in both A and B viruses," Donelan said, referring to the two key categories of flu viruses. "And that's why you need to get vaccinated annually."
She said the majority of people who've been hospitalized at Stony Brook for flu complications had not been vaccinated.
Even though it is January, it is not too late to be vaccinated, Donelan said. The vaccines can take a couple of weeks to work, according to the CDC.
She said one of this season's vaccines is "quadrivalent," which means it protects against four influenza strains: Two A strains and two B strains. It is the first four-component flu vaccine. The usual trivalent vaccine is still available.
Federal health authorities decided to add protection against two strains of B influenza, a change from previous years, which offered two A strains and only one B.
"On a regular basis there had been a B mismatch," Donelan said of the type of B in the vaccine and the strain causing widespread infections.