"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 four million people a day.
"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 northern 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 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 sceptical enough people are immune to significantly slow the spread. But he agreed that changes in Americans' behaviour 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. Over the past two weeks, officials have reported an average of 965 deaths a day from Covid-19, down from 1051 a day in early August.
Deaths from the coronavirus are a lagging indicator — they trail new infections because of the time it takes for people to get sick and succumb to the disease.
The percentage of tests coming back positive for the disease has also declined over the past two weeks, from 7.3 per cent to 6.1 per cent. But that comes as the total number of tests administered has fallen from its August peak of more than 820,000 a day, levelling off in recent weeks at about 690,000 a day.
The situation has improved dramatically in several states that struggled with high caseloads earlier this northern summer.
In Arizona, for example, officials reported 859 new cases today, down from a peak of 5500 in late June. More than 2000 people arrived at the state's hospitals showing symptoms of the virus on a single day in early July. This week, that number has been less than 1000.
In Florida, where more than 10,000 people have died, the state reported 2600 new virus cases today. Earlier in the summer, it was regularly reporting more than 10,000 new cases.
Malinda Coler, 37, of San Francisco, said she has been diligent about mask wearing and other preventive measures, less to protect herself than a best friend who has a compromised immune system, with severe arthritis psoriasis.
"So I wear a damned mask and get infuriated when others don't," she said.
Most states now have some type of mask requirement, either through statewide orders issued by governors or from city and county rules that cover most of their population.
Even some conservative governors have gone along with masks. Mississippi Governor Tate Reeves mandated masks in all public places earlier this month, and Georgia Governor Brian Kemp dropped a lawsuit against Atlanta in a dispute over a requirement by the state's largest city.
Many places around the US are seeing pockets of contagion, especially in college towns where students are holding parties and crowding into bars.
It's not clear what will happen to case numbers as more school districts bring students back to classrooms and colleges reopen their campuses. In recent weeks, schools including the University of North Carolina, Michigan State and Notre Dame have moved instruction online after outbreaks on their campuses.
- AP
