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Flu season is off to a rough start this year, according to the Centers for Disease Control and Prevention. While the virus arrived as expected, cases are rising faster, compared with previous years.

Last week, more than 19,000 patients with influenza were admitted to hospitals, up about 10,000 from the previous week, according to 

. To date, the CDC estimates at least 7.5 million people have been sickened, and over 3,100 people have died from the flu.

The surge seems to be driven primarily by a new strain of the virus — subclade K of influenza A(H3N2) — that emerged in Australia over the summer.

"Anywhere we detect this virus, you can see a large surge of influenza cases coming afterwards," says 

, a virologist at Johns Hopkins Bloomberg School of Public Health. In the U.S., "the timing is not that much different from other flu seasons, but the number of cases, and how quickly those cases are increasing is something that is not usually seen this time of year."

New York has been hit especially hard, with state health officials announcing over 71,000 cases last week — the most cases ever recorded in a single week in the state. But other states are seeing high levels of flu activity, particularly in the northeast, midwest and south.

"The map is mostly red," says Pekosz, indicating high levels of disease that will likely increase over the coming weeks.

"When you're in the middle of seeing the curve start to go up, we just don't have any sense of where it's going to stop," he says. "That's the big concern in most of the medical communities right now."

So far, there's no indication that this new strain is more severe, or even more contagious than previous years, says 

, a virologist at the Icahn School of Medicine at Mount Sinai.

But there have been changes to the virus that may allow it to get around our immune defenses, he says. "There's less immunity to it, and that's allowing the virus to spread very quickly and extensively."

There are some concerns that this season's flu vaccine

 to the new strain, given it emerged after the formulation was decided last February. "I think we're going to have a mismatch between the strain circulating and the vaccine," says Demetre Daskalakis, who led the National Center for Immunization and Respiratory Diseases at CDC until he 

 "But the vaccine is still the best protection we have, even if it's imperfect protection."

Preliminary data from the United Kingdom, which saw an early surge of flu this year, suggests the vaccine is about 

 at preventing hospitalization in adults. "Those numbers are in line with what you would typically see," says Krammer, though he stresses those are preliminary estimates.

Flu vaccines only offer protection if people get them and in the U.S., only 

 have gotten a flu shot this year. That leaves many people unprotected in face of a likely bad flu season, says Daskalakis. He'd like to see the CDC do more to encourage vaccination.

"You're not seeing the robust communication that you would expect," he says. "Usually you'd expect to see more alerts coming out of CDC, more recommendations to be vaccinated."

In response to that criticism, a CDC spokesperson said, "the CDC is strongly committed to keeping Americans healthy during flu season. CDC launched a new national outreach campaign designed to raise awareness and empower Americans with the tools they need to stay healthy during the respiratory illness season," adding "the decision to vaccinate is a personal one. People should consult with their healthcare provider to understand their options to get a vaccine and should be informed about the potential risks and benefits associated with vaccines." 

 In an interview with NPR, Lisa Grohskopf, a medical officer in the CDC's influenza division emphasized the importance of vaccination. "It's definitely not too late to get a flu vaccine if you haven't done it already," she says.

There are other ways of protecting yourself too.

"If you're using public transportation, if you're in the room with a lot of other people, if you're in a healthcare setting, it's really smart to wear a mask," says Krammer, especially higher-quality masks. "I was taking the subway yesterday in New York City. I was wearing an N95 mask."

Social distancing, especially when you or someone in your household is infected, can help minimize the spread too.

If you get infected, there are effective treatment options, especially when started with 48 hours of infection. "If you get an infection with influenza, that's really a reason to see a physician, get diagnosed, and then take next steps," says Krammer. "It's not an infection that you should take lightly."

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Jonathan Lambert

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Gabrielle Emanuel

Flu season is off to a rough start this year, according to new CDC data. The virus is spreading faster than in previous years and the surge is likely to get worse. Here's what you need to know.

Flu cases are surging and rates will likely get worse, new CDC data shows

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