Flu is not done: B strains dominate season’s second wave

The flu season is in its second wave, dominated by influenza B strains as the season enters a downward slope, state and federal data show.

This year’s flu vaccine is 42 percent effective against influenza B, compared with its 25 percent effectiveness against the A strain known as H3N2, which last month was responsible for as many as 4,000 deaths per week nationally. The deaths were occurring mostly among people who succumbed to flu-induced respiratory complications, including pneumonia.

“B is usually worse for younger children,” Kristen Nordlund, spokeswoman for the Centers for Disease Control and Prevention, said in an email Thursday. “We often see an increase in influenza B cases during seasons when influenza A/H3N2 was the predominant virus earlier in the season.”

Nordlund pointed to a CDC study that compared flu caused by B and A strains that found both capable of equally severe forms of disease. That means once infected, there is no way an individual would know which strain had caused the infection. Doctors know the difference only after receiving the results of a lab test.

The CDC has found that B strains of flu are overtaking A strains as the dominant forms of circulating flu nationwide. Throughout winter, flu infections were largely caused by A strains.

“There has certainly been diminished influenza activity in the past three weeks. However, influenza is still circulating,” said Dr. Gary Leonardi, a virologist at Nassau University Medical Center in East Meadow. His lab has seen a shift toward more illnesses caused by influenza B strains than any of the circulating A strains. In addition to H3N2, another one called H1N1 was prevalent throughout peak flu season.

The B strains of influenza only infect humans. A strains infect humans and animals, including birds, pigs, ferrets and bats, among others. Two B strains are circulating in New York, according to the state Department of Health.

Having been infected with influenza A does not provide immunity against influenza B, and it is possible to be infected with both strains at once, researchers have found.

Throughout New York, including Long Island, flu activity remains widespread, even though the overall influenza incidence has declined compared with the peak of the season last month, state Health Department data showed Thursday.

On the Island, flu activity remains intense in pockets of Nassau County, according to the department’s updated flu map. The biggest declines in flu incidence in the two-county region have been in Suffolk County and much of the South Shore.

Statewide, 458 people were hospitalized for flu-like illnesses last week, compared with more than 2,000 per week last month during peak influenza season, according to state statistics.

Health officials Thursday continued to recommend vaccination for those who have not yet gotten a flu shot.

Leonardi’s laboratory, meanwhile, serves as the hub on Long Island for detecting respiratory viruses circulating in the region. He identifies pathogens in NUMC patient samples and reports his findings to the National Respiratory Enteric Viral Surveillance System, a program of the CDC.

That federal program examines the range of viruses, beyond the flu, that are making people sick. The symptoms of multiple but distinctly different viruses often can be mistaken for influenza, Leonardi said.

Viruses isolated among patients at NUMC, a large institution with a diverse patient population, are illustrative of the types of respiratory pathogens causing infections in the region, he said.

In addition to flu pathogens, Leonardi’s lab has detected evidence of adenovirus and rhinovirus, which both cause the common cold and circulate year-round. He and his team also have isolated human metapneumovirus, a relative newcomer among infectious respiratory pathogens, having been discovered in 2001.