A new flu virus circulating this season is proving to be especially resilient, and experts say it appears more infectious than previous strains. Known as subclade K, the virus is a descendant of the familiar H3N2 influenza strain. Health experts say it is also better at evading immunity gained from prior flu infections or vaccination.

The virus was first identified in the United Kingdom, where the current flu vaccine was found to be only 30% to 40% effective at preventing emergency room visits among adults. However, it was up to 70% effective in keeping children out of the ER.

The Centers for Disease Control and Prevention estimates there have been 4.6 million flu cases across the United States so far this season, with at least 1,900 deaths reported, including three children.

“This flu season is no joke. We are seeing more cases than we would expect for this time of year,” said Dr. Amanda Kravitz, a pediatrician at Weill Cornell Medicine in New York, speaking on “CBS Mornings.”

“There are cases throughout the country, and it’s causing a very severe flu,” Kravitz added. “Symptoms are intense and come on very quickly. It’s extremely contagious and spreading rapidly through communities.”

While experts stress that “the flu is still the flu,” this year’s symptoms appear to be more severe and are affecting younger people more intensely than usual. “It can make you quite sick,” said Dr. Thomas Russo, an infectious disease specialist.

Flu symptoms can range from mild to severe and, in some cases, can be life-threatening. According to the CDC, symptoms typically come on suddenly and may include fever or chills, although not everyone develops a fever. Other common symptoms include cough, sore throat, runny or stuffy nose, muscle or body aches, headaches, and extreme fatigue. Some people may also experience vomiting or diarrhea, which is more common in children than in adults.

To help prevent the flu, experts continue to emphasize vaccination and basic precautions.

Dr. William Schaffner, an infectious disease specialist at Vanderbilt University School of Medicine, recommends getting the flu shot if you have not already done so. “It takes about 10 days to two weeks to reach maximum protection,” he says. While the vaccine may not fully prevent infection, it can significantly reduce the risk of hospitalization.

Health experts also advise avoiding close contact with people who are coughing or sneezing and wearing a mask if someone in your household is sick. If you develop flu symptoms, getting tested is important. “We can prescribe antiviral medications such as Tamiflu, which can shorten the duration of illness and may keep you out of the hospital,” Schaffner says.

Staying hydrated and continuing light movement can also help. Dehydration increases the risk of complications, and gentle activity may make breathing easier.

The good news, Schaffner notes, is that this flu season may end earlier than usual. In the meantime, taking preventive steps can help protect you and your loved ones during the peak weeks of illness.