Why this could be the worst flu season in the US in more than a decade

The number of positive flu tests and hospitalizations in the United States is the highest in more than a decade, indicating this influenza season may be the most severe in years.

According to data from the Centers for Disease Control and Prevention, there have been at least 880,000 cases of flu-like illness during the 2022-23 flu season.

Over the season's first three weeks, clinical laboratories have recorded more than 9,100 positive tests, CDC data shows.

This is the highest number of tests recorded this early in a season since 2009-2010, when the country was seeing a swine flu outbreak. More than 21,000 positive tests were recorded that season, according to an ABC News analysis of data from the federal health agency.

Additionally, there have been about 6,900 hospitalizations so far this season with a cumulative rate of 1.5 per 100,000. According to the CDC, this is "higher than the rate observed … during previous seasons going back to 2010-2011."

At least two states have also experienced pediatric flu death deaths within the last week, including two reported in Texas and one in South Carolina.

"What the data tells us so far is we're likely to have a fairly substantial influenza season," Dr. Timothy Brewer, a professor of epidemiology at UCLA Fielding School of Public Health, told ABC News. "It's taking off at a faster rate than usual. We're clearly in the influenza season but what we're seeing is a more rapid rise in cases."

He added, "It does seem to be spreading quicker, particularly along the eastern seaboard and the south. It's unusual for the East Coast to be seeing so much influenza this early in the season."

Public health experts said one of the possible reasons why cases and hospitalizations are so high is because during the COVID-19 pandemic, when schools and businesses were closed and people were staying at home, they weren't exposed to many viruses.

Due to this, people may be more susceptible to getting sick, as their immune systems are remembering (or learning for the first time) how to fight off these infections.

"It takes some time for immunity to build back up," Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center in Boston, told ABC News. "Immunity ... is what protects [us] against severe disease in the future. And that's why we're seeing so many people who are sick in the hospital with viruses."

Doron said there also may be more people being tested for flu this season than usual.

"In a normal flu season, we were not recommended to test everyone with flu symptoms ... we're now in a situation where everyone who has symptoms of any kind is supposed to test."

The doctors stressed it's not too late to get the flu shot and encourage everyone 6 months and older to receive a dose.

"It's definitely not too late," Dr. Jessica Justman, an associate professor of medicine in epidemiology at Columbia Mailman School of Public Health, told ABC News. "Now is a great time. This week is a great time to get your flu shot. I wouldn't put it off, it's still completely worth it."

The experts also suggested Americans follow the same steps they did during the COVID-19 pandemic, including washing hands thoroughly, avoiding crowded indoor spaces and even wearing masks.

"All of those things that we talked about, over and over for COVID, what are called the non-pharmaceutical interventions, they will work really well, to help you avoid flu, just the way they help you avoid COVID," Justman said. "So we've all learned how to do those things. We just have to remember to keep going."