WASHINGTON- 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.

SUMMER OF FLU

The United States will likely continue to see influenza activity through the summer, and at this point we're anticipating that we will see the novel H1N1 continue with activity probably all the way into our flu season in the fall and winter. The amount of activity we expect to be low, and then pick up later.

One worrying pattern: healthcare workers are being infected, and most reported they did little or nothing to protect themselves, the CDC's Dr. Mike Bell said.

People coming into emergency departments or clinics need to be checked right away for flu symptoms and anyone working with such a patient needs to wear a mask, gloves and eyewear, Bell said.

We're beginning to see a pattern of healthcare personnel-to-healthcare personnel transmission in some of the clusters, which is also concerning, because it gets to the issue of people showing up to work sick, Bell said.

Doctors, nurses and technicians who have flu can spread it to vulnerable patients, Bell noted.

As of May 13, the CDC said it had received 48 reports of healthcare workers infected with swine flu.

Detailed case reports on 26 showed that 13 were infected in a healthcare setting such as a clinic or hospital and 12 caught it from infected patients, the CDC said in its weekly report on death and disease.

(Additional reporting by Julie Steenhuysen in Chicago, editing by Philip Barbara)

New York Medicine CDC