The worst of the flu outbreak may be reaching its peak, but influenza viruses are still in abundant circulation as are other winter respiratory microbes that produce flu-mimicking symptoms, experts say.

Surveillance data from the virology department at Nassau University Medical Center show influenza A and B strains are still prevalent throughout the region, with A dominating.

But other viruses, such as parainfluenza virus, human meta-pneumovirus and respiratory syncytial virus -- RSV -- are also on the move.

Of the three, RSV is the most prevalent, according to NUMC virologist Dr. Gary Leonardi, and can be serious in children, especially babies, producing flu-like symptoms and a seal-like, barking cough.

Like the flu, RSV is seasonal, running the same fall-through-spring course. There is no vaccine.

"The flu lasts a few days, but RSV can linger for weeks," Leonardi said, noting the elderly and those with breathing disorders are also vulnerable.

Tests, experts say, have revealed some patients are infected with RSV and influenza. Worse, this season RSV is nearly as prevalent as the A strain of flu that's linked to the most infections.

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"We are seeing almost as many positive RSV tests as we are positive ones for influenza A," said Dr. Susan Donelan, medical director for health care epidemiology at Stony Brook University Hospital. "And, we are seeing them in just about every age group."

The problem, Donelan added, is people can't tell without the benefit of a test whether their respiratory infection is the flu, another seasonal pathogen -- or both.

Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, said last week the current flu season is "worse than average" and is similar to the 2003-2004 flu outbreak in intensity. There's evidence, Frieden said, the season may have reached its peak.

Still, in New York, the public health emergency declared by the governor two weeks ago because of the flu remains in effect, said Peter Constantakes, spokesman for the New York State Department of Health.

To date, 4,153 New Yorkers have been hospitalized with the flu since the season's early start in September and three children have died. The state tracks neither adult deaths nor the incidence of RSV, Constantakes said.

The state laboratory has confirmed 23,501 flu cases, Constantakes added, noting the count refers only to people who visited a doctor who took a throat culture.

Donelan, meanwhile, has noticed somewhat fewer patients in recent days coming to Stony Brook complaining of influenza symptoms.

"Fortunately, it seems to be slowing down a little bit, which we appreciate," she said.

Dr. Luis Rodriguez, a member of the American Lung Association's Leadership Board, said even though it's late January, it's still not too late to be vaccinated against the flu.

"It's your best defense," he said.