MIAMI- About half of the US -- 22 states, along with Washington, DC, New York City and Puerto Rico -- is reporting high or very high respiratory illness activity, as flu season sweeps through the country weeks earlier than usual.

Multiple respiratory viruses are circulating nationwide -- including flu, RSV and the virus that causes Covid-19 -- and the US Centers for Disease Control and Prevention tweeted Thursday that flu is contributing to a "significant proportion" of that circulation.

And influenza activity continues to increase: After nearly doubling in the last week of October, the number of flu illnesses, hospitalizations and deaths this season took another significant jump in the first week of November. The CDC now estimates that there have been at least 2.8 million illnesses, 23,000 hospitalizations and 1,300 deaths from influenza as of November 5.

The flu is notoriously hard to predict.

"Among the people who study flu, there's a little saying: 'If you've seen one flu season, well, you've seen one flu season,' " said Dr. William Schaffner, a professor in the Division of Infectious Diseases at Vanderbilt University Medical Center and medical director of the National Foundation for Infectious Diseases. "The implication is that they're all a little bit different from each other, and that's certainly the case."

This season, there have been about 5 flu hospitalizations for every 100,000 people, rates typically seen in December or January, CDC data shows. And in the most recent week, about 13% of lab tests -- more than 1 in 8 -- were positive for influenza, up from 9% the week before.

Although the numbers are especially high for this time of year, experts say the trends are otherwise following an expected pattern, and the early arrival of flu season doesn't necessarily mean it will last longer or be more severe.

"The picture is pretty consistent across our different pieces of surveillance. There's nothing there that makes me think that this virus is really different and causing more severe disease than we see typically with flu," said Lynnette Brammer, an epidemiologist who leads the CDC's domestic influenza surveillance team. "Flu can cause severe outcomes, but it's not out of proportion this year compared to previous years. It's not like we're seeing a lot of hospitalizations without a lot of illness."

Flu activity has been highest in the South, CDC data shows. Data from Walgreens that tracks prescriptions for antiviral treatments such as Tamiflu suggests hotspots centered in Mississippi and Alabama, spreading from the Gulf Coast area, including Houston and New Orleans, up to Nashville.

As flu smolders in the South, RSV is showing early signs of slowing in the region, CDC data shows, even as it continues to ramp up in other parts of the country.

Nationwide, nearly 1 in 5 PCR tests for RSV were positive in the first week of November, reaching levels that the South saw a month ago. But in that region, fewer than 1 in 7 tests are now positive for RSV.

RSV is a common respiratory virus that can cause serious illness, especially in younger infants, and it's kept pediatric hospitals more full than usual in recent weeks.

Despite the improvements in the South, overall burden on pediatric hospitals nationwide has not eased. More than three-quarters of pediatric hospital beds and pediatric ICU beds have been in use for the past few weeks -- up from an average of about two-thirds full over the past two years, according to federal data.

On Thursday, Dr. Brian Cummings, medical director of the Department of Pediatrics at Mass General for Children, said they've seen around 2,000 cases of RSV in October, and more than 1,000 in the first week of November.

"It's been escalating and been quite severe," Cummings said during a news conference Thursday.

Most infections have been treated in urgent care facilities and in the emergency department, and people are sent home, Cummings said. "But even if just 10 percent of those need hospitalization, it creates a lot of stress on health care facilities, and so what we are seeing is we've had over 250 hospitalizations for RSV alone on top of the other circulating viruses," he said.

The pediatric ICU is full, Cummings said, and seven patients are waiting to be transferred in.

Across the country, the threat of Covid-19, while significantly less than it was a year ago, remains.

Nearly 25,000 people were admitted to the hospital with Covid-19 in the first week of November, according to CDC data, a pace that's forecast to hold steady for the next month, at least.

As multiple respiratory viruses continue to spread, experts emphasize the importance of vaccination, which is available for two of the three viruses in heavy circulation. But those numbers are lower than ideal.

One in 5 people still hasn't gotten their first Covid-19 vaccine shot, CDC data shows, and less than 10% of the US population has gotten the updated Covid-19 booster.

And millions fewer flu vaccines have been distributed this season than at this point in previous years.

"You worry when you have an early season that people just haven't had time to get vaccinated," Brammer said. "But vaccine is the best tool that we have to prevent these severe outcomes."

By Deidre McPhillips, CNN 

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