Public health officials are warning that the U.S. may be on the verge of a dangerous double whammy: COVID and flu, spreading simultaneously.

The big picture: The Delta variant is still circulating across the U.S., and the Omicron variant isn't far behind. On top of that, experts see potential warning signs of a bad flu season, which could leave millions of Americans vulnerable and strain health care resources.

What they're saying: "If people resume 'life as normal' without masking and without getting flu shots or COVID shots, then I think we're in for a tough winter," said Gregg Miller, the chief medical officer of Vituity, a firm that staffs hospital emergency departments.

• "Many of us have been concerned this year may be a worse influenza season, but it's too early to say," Céline Gounder, an infectious disease specialist and epidemiologist at New York University and Bellevue Hospital told Axios.

By the numbers: Just 60% of Americans are fully vaccinated against COVID.

• 41% of adults have gotten a flu shot, according to the CDC, as have 39% of children — significantly lower than the same point last year.

State of play: The CDC has said flu activity is still low nationwide, but recently issued a warning to providers about the early circulation of a strain of flu known as influenza A(H3N2).

• Previous seasons in which that virus was the predominant strain saw more hospitalizations and deaths among people 65 and older. The viruses also evolve more rapidly to escape human immunity, the CDC said.

Between the lines: A bad flu season could further exacerbate the health disparities highlighted by the pandemic.

• Vaccination rates tend to be lower among Black and Latino Americans than white Americans, per the CDC.

The bottom line: "Overall health care capacity is the biggest concern. If it turns out we have a bad flu season, that begins to overwhelm urgent care and primary care, in addition to what could be another surge of COVID," said Michael Ganio, senior director of pharmacy practice and quality at the American Society of Health-System Pharmacists.

• "So obviously the number one recommendation for all of us is vaccination," he said.