New coronavirus seven-day case averages have fallen in 43 states, and hospitalizations have dipped in 49, according to a new report.

The downward trend in cases is now in its fifth week, according to the Centers of Disease Control and Prevention. The only five states reporting a rise in case averages include Nebraska, New Hampshire, North Dakota, Vermont and Wyoming. Washington was the only state that did not have a decline in hospitalizations, and its seven-day rate was stable over the prior week, according to a CNBC analysis of Johns Hopkins University data. 

Meanwhile, COVID-19 test volume has fallen, with the latest February data showing a 16% decline from the week prior, the agency reported.

“[C]ase numbers really are falling — it’s not just reduced testing — and it’s happening pretty much everywhere,” said infectious disease expert Paul E. Sax, M.D., of Harvard. This includes urban and rural areas in the United States, globally no matter the state of vaccination campaigns, and in places with a high level of the highly contagious B117 variant, he wrote in a Sunday blog post in The New England Journal of Medicine

Although there it is unclear what is driving the trend, it is likely due to a mix of factors, he wrote. These include:

Vaccination: Although vaccinations alone can’t explain the falling case rates, demand is “off the charts” worldwide, and most places are targeting people most likely to have symptomatic or severe disease, Sax said. Vaccines may reduce the likelihood of transmission along with infections overall, he said. 

Herd immunity: The almost 28 million Americans with a confirmed COVID-19 diagnosis represent only a fraction of true cases, Sax said. Considering federal estimates that only one in 4.6 infections are reported, as much as half the U.S. population may have been exposed, “with some degree of natural immunity to infection.”

In mid-January, the CDC’s estimate of actual case numbers was more than 80 million. “And note that in some regions, the actual case counts might be even higher — five to 20 times higher,” Sax added.

Behavior: One hypothesis related to herd immunity is that the people least likely or unable to follow infection control advice already have had COVID-19 and are immune, and those who do follow mitigation measures are managing to stay safe.

The virus is becoming less virulent: “Maybe the virus is doing us a favor and becoming less virulent over time. Perhaps some of these variants — if not B117 — in order to gain the ability to transmit, also cause less severe disease,” Sax wrote.

It remains to be seen whether the case count will continue to fall, he concluded. “[I]f there’s one thing a pandemic from a new human disease teaches us, it’s that there’s a lot we don’t know.”