U.S. health officials have warned that respiratory virus activity is on the rise, with overall activity trending at moderate levels across the United States.

Around the United States, test positivity for influenza increased to 9 percent, COVID-19 "increased slightly" to 5.6 percent, and RSV positivity increased to 9.5 percent, the CDC said.

For the flu, the virus has "been increasing at a pretty steady pace now for the past several weeks," said CDC researcher Alicia Budd on Dec. 20. "We are certainly in flu season now," she added.

Thirteen states reported high or very high levels of flu-like illness last week, about double from the week before. One is Tennessee, where a sickness spike is hitting the Nashville area, said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University.

"Flu has been increasing but just this last week has exploded," Schaffner said. He noted that in a local clinic that serves as an indicator of illness trends, as many as a quarter of the patients have flu symptoms.

Overall, RSV activity is moderate nationally "and continues to increase in most areas of the United States, particularly in young children," the agency said.

The CDC called the start of flu season based on several indicators, including lab results for patients in hospitals and doctor's offices, and the percentage of emergency department visits that had a discharge diagnosis of flu.

No flu strain seems to be dominant, and it's too early in the season to know how good of a match the flu vaccine will be, Budd said.