A second wave of coronavirus cases in the US could be even worse than the first, the country's top health official has warned.
Centers for Disease Control and Prevention (CDC) director Robert Redfield said the danger was higher as a fresh outbreak would likely coincide with the flu season.
It would put "unimaginable strain" on the US health care system, he said.
The US has seen more than 800,000 cases - the highest in the world.
More than 45,000 people have so far died with coronavirus across the US, according to a tally kept by Johns Hopkins University.
California had its highest one-day rise in new cases on Monday while New Jersey, the worst-hit US state apart from New York, saw its highest increase in deaths in one day.
In other developments in the US:
President Trump says he will halt applications for US green cards - which give immigrants legal permanent residence and the opportunity to apply for American citizenship - for 60 days
Harvard University says it will keep an $8.6m-coronavirus grant despite pressure from President Trump to return it California's first cases of coronavirus occurred much earlier than previously thought, health officials have confirmed. A post-mortem examination has revealed that a person who died at home on 6 February in Santa Clara county is now the first known fatality in the US. Until now, the first fatality was thought to have been a man in Washington state who died on 26 February
The US Senate has unanimously approved $484bn (£392bn) in coronavirus relief, including funds designed to help small businesses. The legislation will go to the House of Representatives for approval on Thursday
In an interview with the Washington Post, Mr Redfield said that "there's a possibility that the assault of the virus on our nation next winter will actually be even more difficult than the one we just went through".
He urged officials in the US to prepare for the possibility of having to confront a flu and a coronavirus epidemic at the same time.
'Really, really difficult' Mr Redfield stressed the importance of getting flu shots. He said that getting vaccinated "may allow there to be a hospital bed available for your mother or grandmother that may get coronavirus".
The CDC chief said that coronavirus had arrived in the US as the regular flu season was subsiding. He argued that if it had arrived at the peak of the winter influenza season, "it could have been been really, really, really, really difficult".
WHEN WILL A SECOND WAVE OF THE CORONAVIRUS HIT, AND WHAT WILL IT LOOK LIKE? - USA TODAY
Even before the first horrific phase of the COVID-19 pandemic has run its course, scientists are worried about the second wave of the disease.
It could crash worse than the first, killing tens of thousands of people who did such a good job of sheltering in place they remain virgin ground for the virus. Or it could be a mere swell, with so many people having been infected without symptoms that levels of immunity are higher than realized.
There is no crystal ball to look to, because so many crucial pieces of information remain missing.
Are people who've had COVID-19 immune? How long does immunity last? Will the virus play out like influenza and the common cold, peaking during cooler months and falling during warmer ones? Is its deadly path undeterred whatever the weather?
Until there’s a vaccine “it’s unfortunately not unlikely that we may see a second wave or even a third wave,” said Peter Marks, director of the U.S. Food and Drug Administration’s Center for Biologics Evaluation and Research, which oversees vaccines. He spoke on a call with reporters about vaccine development organized by the Commonwealth Fund.
“I shudder to think of that, but I think we have to be realistic."
Immunity: Can you get the coronavirus more than once?
The first question on every doctor’s mind is whether someone who has had COVID-19 is immune, and if so for how long. People who've had mumps are immune to mumps for life. Versions of the common cold caused by different types of coronavirus see immunity wane within a year. Variations are wide.
COVID-19 is such a new disease that there is no solid data on the immunity of survivors. But given its similarities to coronaviruses such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), it's expected to convey at least some immunity.
That’s good news because so many people have contracted COVID-19 from SARS-CoV-2, the virus that causes the disease. Many more probably have had it asymptomatically and didn’t know. Both groups likely have some immunity.
What percentage of the population is immune is impossible to know because the United States doesn’t yet have widespread testing, experts say. An answer is likely months away. Even if immunity isn't lifelong, the virus may have infected enough people to make it hard for it to find new victims.
“It would mean we’d be getting to herd immunity through natural immunity” even before a vaccine is developed, said Marc Lipsitch, a professor of epidemiology at Harvard’s Chan School of Public Health, who co-wrote a paper in the journal Science modeling what COVID-19 might look like after the initial pandemic has passed.
