In recent weeks, U.S. coronavirus case data — long a closely-watched barometer of the pandemic’s severity — has sent some encouraging signals: The rate of newly recorded infections is plummeting from coast to coast and the worst surge yet is finally relenting. But scientists are split on why, exactly, it is happening. Some point to the quickening pace of coronavirus vaccine administration, some say it’s because of the natural seasonal ebb of respiratory viruses and others chalk it up to social distancing measures.

And every explanation is appended with two significant caveats: The country is still in a bad place, continuing to notch more than 90,000 new cases every day, and recent progress could still be imperiled, either by new fast-spreading virus variants or by relaxed social distancing measures.

A former director of the Centers for Disease Control and Prevention endorsed the idea that Americans are now seeing the effect of their good behavior — not of increased vaccinations.

“I don’t think the vaccine is having much of an impact at all on case rates,” Tom Frieden said in an interview Sunday on CNN’s “Fareed Zakaria GPS.” “It’s what we’re doing right: staying apart, wearing masks, not traveling, not mixing with others indoors.”

However, Frieden noted, the country’s numbers are still higher than they were during the spring and summer virus waves and “we’re nowhere near out of the woods.”

The current CDC director, Rochelle Walensky, said in a round of TV interviews Sunday morning that behavior will be crucial to averting yet another spike in infections and that it is far too soon for states to be rescinding mask mandates. Walensky also noted the declining numbers but said cases are still “more than two-and-a-half-fold times what we saw over the summer.”

“It’s encouraging to see these trends coming down, but they’re coming down from an extraordinarily high place,” she said on NBC’s “Meet the Press.”

Researchers at the University of Washington’s Institute for Health Metrics and Evaluation, publisher of a popular coronavirus model, are among those who attribute declining cases to vaccines and the virus’s seasonality, which scientists have said may allow it to spread faster in colder weather.