Flu season is starting to ramp up as the U.S. settles into the colder weather months and indoor gatherings potentially fuel the spread of viruses.

At least four states, including Colorado, Louisiana, New Jersey and New York, are experiencing high levels of influenza-like illness as of the week ending Dec. 6, according to data from the Centers for Disease Control and Prevention (CDC).

During the same period, 8.1% of weekly tests were positive for influenza, about four times higher than the 1.6% of weekly tests that were positive one month earlier, CDC data shows.

More than 6,800 patients were admitted to hospitals with flu for the week ending Dec. 6. What’s more, the first pediatric death associated with the flu was reported by CDC during the same week.

Meanwhile, there is some concern about a newer flu strain that may have an ability to evade immunity better than other strains.

Here’s what you need to know about the current flu season.

What does flu season look like so far? 

Although flu season has begun, it’s not yet in “full form,” according to Dr. Peter Chin-Hong, a professor of medicine and infectious disease specialist at the University of California, San Francisco.

He told ABC News that flu cases typically peak in January or February, so the U.S. still has some time before it might see the worst of flu season.

However, he did express some concern that states such as New York may be experiencing its flu season earlier than expected. 

Data from the New York State Department of Health shows the state recorded more than 24,000 flu cases and nearly 1,400 hospitalizations for the week ending Dec. 6. During the same week last year, the state recorded more than 12,000 cases and around 850 hospitalizations. 

“If we look at what’s happening around the world, for example, the U.K., and what has happened in Japan and other parts of Asia, we’re worried that this may be yet another bad flu season,” Chin-Hong said. 

How much is subclade K playing a role? 

Public health experts told ABC News a new influenza strain was a main driver of a spike in flu cases in Canada, Japan and the U.K.

The new flu strain, called subclade K, is a variant of the H3N2 virus — a subtype of influenza A — that has been circulating since the summer in other countries. 

Among 163 samples of H3N2 viruses collected since Sept. 28 and genetically characterized, 89% were subclade K, according to the CDC. 

Dr. Tony Moody, a professor of pediatrics and immunology at Duke University, said there is not yet firm data on whether subclade K causes more severe disease or how immune systems are responding to the new strain, but it does seem to be better at evading immunity. 

“We think that this new circulating dominant strain probably is one that has just found a little bit of a hole,” he told ABC News. “In some ways. It’s almost like a NASCAR race, right? It found a slot that it could zoom through. But it’s going to take time to get firm data on that and really be able to say that for absolute sure.” 

Chin-Hong said data from the U.K. shows that the flu vaccine is still very effective at preventing severe disease despite the emergence of subclade that causes  

A pre-print from the U.K. Health Security Agency shows the 2025-26 vaccine is 70 to 75% effective at preventing hospitalization in children between ages 2 and 17 and 30 to 40% effective in adults. 

“To me, even though they are about seven new mutations in subclade K, it means that if you get the vaccine, you’re going to diminish your chances of getting seriously ill,” Chin-Hong said. “But if you don’t get the vaccine, because there’s so many [new mutations], you may get sick more easily compared to, say, even last year.” 

How to best protect yourself  

Currently, the CDC recommends that everyone 6 months and older, with rare exceptions, get a flu vaccine.  

The federal health agency states on its website that getting an annual flu vaccine is the best way to reduce the risk of infection and potentially serious complications. 

“Fundamentally, getting vaccinated is still one of the best ways that we know of to cut down on the severity of disease,” Moody said. “The vaccines are not perfect … but we do know from study after study that people who get vaccinated have a lower risk of death.” 

He continued, “People who get vaccinated tend to have a lower risk of being admitted to the hospital or going to the intensive care unit. So, it’s still a good way to try to cut down on influenza risk.” 

Additionally, Moody and Chin-Hong said there are mitigation measures people can take, including washing hands thoroughly with soap and water, and covering a cough or a sneeze with a hand or elbow. 

They added to consider wearing a mask if in a crowded indoor space and to not travel if you fall ill. 

Chin-Hong said for those who suspect they may have the flu — or are sick and unsure if they have the flu — to get an official diagnosis. 

“Don’t forget that getting a diagnosis is really crucial when you have the sniffles because, if it’s flu, you may be eligible for antivirals like Tamiflu, particularly this year when you know you may not have gotten the vaccine yet, the antiviral can really help just like we’ve seen in other respiratory diseases,” he said.