WASHINGTON — The number of Americans newly diagnosed with the coronavirus is falling — a development experts say most likely reflects more mask-wearing but also insufficient testing — even as the disease continues to claim nearly 1,000 lives in the US each day. About 43,000 new cases are being reported daily across the country, down 21 percent from early August, according to data compiled by Johns Hopkins University. While the US, India and Brazil still have the highest numbers of new cases in the world, the downward trend is encouraging. “It’s profoundly hopeful news,” said Dr. Monica Gandhi, an infectious-diseases expert at the University of California, San Francisco, who credits the American public’s growing understanding of how the virus spreads, more mask-wearing and, possibly, an increasing level of immunity. “Hopefully all those factors are coming into play to get this virus under control in this country that’s really been battered by the pandemic,” she said. But insufficient testing is probably concealing the full extent of the crisis, said Dr. Jonathan Quick, who leads the pandemic response for the Rockefeller Foundation, which has recommended the US test 4 million people a day by fall. “We’re grossly under-testing in some of the places that are still having high caseloads,” Quick said, singling out Mississippi, Texas, Georgia and North Dakota as hot spots with high rates of positive test results. Even at 43,000 new cases per day, the US remains far above the numbers seen during the spring, when new daily cases peaked at about 34,000, he said. “It’s a good trend, but nowhere near what we need to be,” Quick said of the recent decline. The virus is blamed for more than 5.7 million confirmed infections and about 178,000 deaths in the US Worldwide, the death toll is put at more than 810,000, with about 23.7 million cases. Jeffrey Shaman, a public health expert at Columbia University, said he is skeptical enough people are immune to significantly slow the spread. But he agreed that changes in Americans’ behavior could well be making a difference, recalling the impact that people’s actions had in containing Ebola in West Africa several years ago. “Ebola stopped for reasons we didn’t anticipate at the time. It was so horrifying that people stopped touching each other,” Shaman said. Something similar may be happening with the coronavirus, he said. “I know I don’t have nearly the number of contacts that I used to,” Shaman said. “But if we relax that, if we get complacent, will we just see another outbreak?” The decline in newly reported cases in the US comes even as deaths from the virus remain alarmingly high. Officials have reported an average of 965 deaths per day from COVID-19 recently, down from 1,051 deaths a day in early August. “It’s not just about football. So, for people to make the right choices and decisions to wear their masks, do the things when they’re out publicly, respect the rules, respect the virus, that’s important,” he said. — Agencies