Leading US public health expert and White House coronavirus taskforce member Dr Anthony Fauci has said the US may not see a “second wave” of cases of Covid-19.

According to researchers at Johns Hopkins University in Maryland, the US has recorded more than 2m cases of Covid-19 and nearly 115,000 deaths.

Related: Coronavirus map of the US: latest cases state by state

Many experts fear attempts to reopen shuttered state economies and mass protests over police brutality and structural racism could contribute to a second surge in cases.

But Fauci, the director of the National Institute of Allergy and Infectious Diseases who has been sidelined by the White House since April, after breaking with Donald Trump’s position on reopening the economy, told CNN on Friday increases in cases in several states were not necessarily indicative of a “second spike” of infections.

“When you start to see increases in hospitalisation, that’s a surefire situation that you’ve got to pay close attention to,” Fauci said.

“It is not inevitable that you will have a so-called ‘second wave’ in the fall, or even a massive increase if you approach it in the proper way,” he added, advising people to maintain social distancing and to continue to wear masks in public.

According to the federal Centers for Disease Control and Prevention (CDC), close to 80% of Americans self-isolated in the last month and 74% wore face coverings in public either always or often. Residents of New York and Los Angeles did so about 90% of the time.

In the past week, 19 states including Texas, South Carolina, Utah, Arizona, North Carolina, Arkansas, Alabama, Oregon, California, Nevada and Florida have reported seven-day rolling average highs for new Covid-19 infections.

In Arkansas on Friday, for example, Governor Asa Hutchinson, a Republican, announced a record number of cases in the previous 24 hours. In Oregon, Governor Kate Brown, a Democrat, placed a seven-day hold on lifting on loosening social restrictions.

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In Maryland, Baltimore announced that it would not be moving into phase two of planned reopening.

In a statement, the mayor, Bernard Young, said: “Let me be crystal clear with everyone: I, more than almost anyone, would love to see that Baltimore city is open and safe, but that simply is not what the data is telling us at this time.”

The CDC said it could it not confirm reports of striking increases in coronavirus hospitalisations but would monitor the numbers “very closely”.

Such developments sent stock markets into a tailspin on Thursday, before a slight recovery on Friday.

Some experts, meanwhile, have become more optimistic that a vaccine for Covid-19 can be developed swiftly.

Related: Dr Amy Acton resigns amid backlash against Ohio’s lockdown after leading coronavirus fight

“I think the science is on our side,” the former CDC director Julie Gerberding told CNN.

But she cautioned: “That doesn’t say anything about the speed, the safety and the durability and all of the other criteria that have to come into play before we have something that we can count on to give us that population immunity.”

In New York City, mass protests have sparked fears of a resurgence of the virus in the American pandemic center. But new infections are now at the lowest level in the entire US.

“We were the No 1 state in terms of infection … and now we are the last state in terms of rate of transmission,” New York’s governor, Andrew Cuomo, said on Friday. “That is because New Yorkers stepped up. They were smart. They were disciplined. They did what they had to do, and we need to stay there.”