The new strain of H1N1 flu is causing “something different” to happen in the United States this year — perhaps an extended year-round flu season that disproportionately hits young people, health officials said on Thursday.

An unusually cool late spring may be helping keep the infection going in the U.S. Northeast, especially densely populated areas in New York and Massachusetts, the officials at the U.S. Centers for Disease Control and Prevention said.

And infections among healthcare workers suggest that people are showing up at work sick — meaning that workplace policies may be contributing to its spread, the CDC officials said.

The new strain of swine flu is officially a pandemic now, according to the World Health Organization.

So far the virus is causing mild to moderate disease, but it has killed at least 167 people and been confirmed in nearly 40,000 globally.

The United States has been hardest hit, with upward of 100,000 likely cases and probably far more, with 44 deaths and 1,600 hospitalized.

“The fact that we are seeing ongoing transmission now indicates that we are seeing something different,” the CDC’s Dr. Daniel Jernigan told a news briefing.

“And we believe that that may have to do with the complete lack of immunity to this particular virus among those that are most likely affected. And those are children,” Jernigan added.

“The areas of the country that are most affected, some of them have very high population densities, like Boston and New York. So that may be a contributor as well. Plus the temperature in that part of the country is cooler, and we know that influenza appears to like the cooler times of the year for making transmission for effective.”

Jernigan said in areas that are the most affected up to 7 percent of the population has influenza-like illness.

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