Flu activity is rising sharply across the United States, according to new data from the Centers for Disease Control and Prevention.

The CDC estimates there have been at least ​7.5 million illnesses, ​81,000 hospitalizations and ​3,100 deaths from flu so far this season. ​Five more pediatric deaths were reported this week, bringing the total to ​eight this season.

Public health experts previously told ABC News that many of this season's cases are linked to a new flu strain called subclade K -- a variant of the H3N2 virus, which is itself a subtype of influenza A.

Subclade K has been circulating since the summer in other countries and was a main driver of a spike in flu cases in Canada, Japan and the U.K.

Flu cases on the rise amid holiday travel, gatherings, latest CDC data shows

Of the 1,600 flu samples tested by the CDC, roughly ​92% were H3N2. Of those samples, nearly ​90% belonged to subclade K.

At least 20 states are now seeing "very high" respiratory illness activity including Arkansas, Colorado, Connecticut, Georgia, Idaho, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, Rhode Island, South Carolina and Tennessee.

Meanwhile, nine states are seeing "high" activity including Alabama, California, Florida, Indiana, Kansas, Maine, Nevada, Texas and Virginia.

Experts expect flu-like illnesses to continue to climb in the coming weeks after holiday gatherings and colder weather.

Outpatient visits for respiratory illnesses were largely among children and young adults. ​About 27% of outpatients visits around the country were among people less than 24 years old experiencing a respiratory illness and children less than five made up ​15% of those.

In addition to influenza, COVID-19 and respiratory syncytial virus (RSV) are also widely circulating this time of the year and contribute to respiratory illness activity.

New York health officials recently announced that the state saw the highest number of flu cases ever recorded in a single week.

Nearly 130 million doses of the flu shot have been distributed so far this season, compared to over 143 million this same time last year. Despite the flu vaccine being widely available, only about 42% of adults and children have received the shot this season.

The mutations seen in the new variant result in a mismatch with this season’s flu vaccine composition, the CDC says. Experts still believe that the flu vaccine will help reduce the risk of severe illness, including hospitalization and death.

Doctors are urging everyone who is eligible for a flu shot to get one this season, emphasizing it’s not too late.

Flu cases are on the rise in the US. Here's what you need to know

Last season, 288 children died from the flu and nearly all were unvaccinated, according to a CDC study. This was the same number of kids who died from the virus during the 2009 H1N1 pandemic and the highest number of deaths ever recorded since tracking pediatric deaths became mandatory in 2004.

Public health experts say other ways to slow the spread of respiratory illnesses include good hand washing, staying home if you’re sick or avoiding people who are sick, covering a cough or sneeze with a tissue that is thrown immediately in the trash and cleaning or disinfecting objects or surfaces that are frequently touched.

Jade A. Cobern, MD, MPH, is a practicing physician, board-certified in pediatrics and general preventive medicine, and is a medical fellow of the ABC News Medical Unit.