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.

The rolling daily average of new infections in the United States hit its all-time high of 248,200 on Jan. 12, according to data gathered and analyzed by The Washington Post. Since then, the number has dropped every day, hitting 91,000 on Sunday, its lowest level since November.

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.”

“We’ve had three surges,” Frieden said. "Whether or not we have a fourth surge is up to us, and the stakes couldn’t be higher.”

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.

In the IHME’s most recent briefing, published Friday, the authors write that cases have “declined sharply,” dropping nearly 50 percent since early January.

“Two [factors] are driving down transmission,” the briefing says. “1) the continued scale-up of vaccination helped by the fraction of adults willing to accept the vaccine reaching 71 percent, and 2) declining seasonality, which will contribute to declining transmission potential from now until August.”

The model predicts 152,000 more covid-19 deaths by June 1, but projects that the vaccine rollout will save 114,000 lives.

In the past week, the country collectively administered 1.62 million vaccine doses per day, according to The Washington Post’s analysis of state and federal data. It was the best week yet for the shots, topping even President Biden’s lofty goal of 1.5 million vaccinations per day.

Nearly 40 million people have received at least their first dose of a coronavirus vaccine, about 12 percent of the U.S. population. Experts have said that 70 percent to 90 percent of people need to have immunity, either through vaccination or prior infection, to quash the pandemic. And some leading epidemiologists have agreed with Frieden, saying that not enough people are vaccinated to make such a sizable dent in the case rates.

A fourth, less optimistic explanation has also emerged: More new cases are simply going undetected. On Twitter, Eleanor Murray, a professor of epidemiology at Boston University School of Public Health, said an increased focus on vaccine distribution and administration could be making it harder to get tested.

“I worry that it’s at least partly an artifact of resources being moved from testing to vaccination,” Murray said of the declines.

The Covid Tracking Project, which compiles and publishes data on coronavirus testing, has indeed observed a steady recent decrease in tests, from more than 2 million per day in mid-January to about 1.6 million a month later. The project’s latest update blames this dip on “a combination of reduced demand as well as reduced availability or accessibility of testing.”

“Demand for testing may have dropped because fewer people are sick or have been exposed to infected individuals, but also perhaps because testing isn’t being promoted as heavily,” the authors write.

They note that a backlog of tests over the holidays probably produced an artificial spike of reported tests in early January, but that even when adjusted, it’s still “unequivocally the wrong direction for a country that needs to understand the movements of the virus during a slow vaccine rollout and the spread of multiple new variants.”

Where most experts agree: The mutated variants of the virus pose perhaps the biggest threat to the country’s recovery. One is spreading rapidly and another, known as B.1.351, contains a mutation that may help the virus partly evade natural and vaccine-induced antibodies.

Fewer than 20 cases have been reported in the United States, but a critically ill man in France underscores the variant’s potentially dangerous consequences. The 58-year-old had a mild coronavirus infection in September and the B.1.351 strain reinfected him four months later.

No matter what’s causing the current downturn in new infections, experts have urged Americans to avoid complacency.

“Masks, distancing, ventilation, avoiding gatherings, getting vaccinated when eligible. These are the tools we have to continue the long trip down the tall mountain," Caitlin Rivers, an epidemiologist at Johns Hopkins University, said on Twitter. "The variants may throw us a curve ball, but if we keep driving down transmission we can get to a better place.”

Jacqueline Dupree contributed to this report.