Centers for Disease Control and Prevention director Robert Redfield warned in an interview with the Washington Post Tuesday that the second wave of the coronavirus this winter could be even more deadly due to its alignment with the start of flu season.

Why it matters: Redfield urged state and federal officials to use this time to prepare by ramping up testing capacity and contact tracing. He also stressed the need for Americans to understand the importance of social distancing as states lift stay-at-home orders, calling protests against the restrictions "not helpful."

Simultaneous respiratory outbreaks would heighten the demand for medical resources like ventilators and protective equipment, putting a tremendous strain on the health care system.

Annual flu shots will also be increasingly important in order to minimize hospitalizations.

What he's saying: "There’s a possibility that the assault of the virus on our nation next winter will actually be even more difficult than the one we just went through. And when I’ve said this to others, they kind of put their head back, they don’t understand what I mean," Redfield told the Post.

Flashback: The 2009 H1N1 swine flu pandemic had its first wave in the spring, but experienced a larger second wave in the fall and winter.

The big picture: Redfield acknowledged the need for a massive testing and tracing effort, which public health experts agree is crucial for being able to reopen large parts of the country.