The United States' top infectious disease physician has warned senators that the virus will spread if the country opens too soon.
Anthony Fauci said that if federal guidelines for reopening are not followed, "small peaks" will become outbreaks.
He also said that the actual number of deaths in the United States is probably higher than the official number of 80,000.
His message is at odds with the optimistic tone of President Trump, who is eager to resume the economy.
Fauci was speaking by video to a US Senate Republican committee.
He was referring to the White House Opening America again plan, which includes three 14-day phases that states should consider implementing, as they allow schools and businesses to reopen.
Several US states that are already restarting their economies have infection rates that are increasing, not falling.
He warned of the risk of triggering an outbreak that authorities would not be able to control, adding that the outbreak would delay economic recovery and could lead to "suffering and death".
While the White House has established guidelines for reopening, it is ultimately up to state governors to make decisions about how to facilitate the blockade.
"There is no doubt, even in the best of circumstances, when you step back from mitigation, you will see some cases come up," Fauci warned American officials.
& # 39; Cautiously optimistic & # 39;
Asked about the possibility of a fall recovery from the virus, Fauci said that one is "entirely conceivable and possible".
"I hope that if we have the threat of a second wave, we can deal with it very effectively, to prevent it from becoming an outbreak."
Fauci also said that there are several vaccines under development, but "no guarantees" will be effective, although, based on his knowledge of other viruses, he is "cautiously optimistic".
"We have many candidates and we hope to have several winners," he said. "In other words, there are several shots on goal."
Witnesses testify remotely
Witnesses appeared remotely during the three-hour hearing, as did some senators on the Senate Health, Educational Work and Pensions Committee.
The three members of the White House coronavirus task force who testified isolated themselves after possible exposure to the disease, including Dr. Fauci.
The director of the Center for Disease Control and Prevention, Dr. Robert Redfield, and Commissioner for Food and Drug Administration, Stephen Hahn, are also self-isolating.
Dr. Fauci has tested negative, but will continue to work from home for now and will be tested regularly.
The audience was led by Tennessee Republican Senator Lamar Alexander from his home in the Smoky Mountains. His dog Rufus was occasionally seen sleeping in the background of the video conference.
"Staying at home indefinitely is not a solution to this pandemic," said Alexander, adding, "There is not enough money available to help everyone affected by a closed economy."
Washington Senator Patty Murray, the highest-ranking Democrat on the committee, said Trump was more interested in "fighting the truth than fighting the virus".
The government cannot ask people to start their lives over "if there is no clear and detailed guidance on how to do this safely," she adds.
Meanwhile, Vice President Mike Pence also keeps his distance from the president after his press secretary Katie Miller tested positive last week.
"The vice president decided to keep his distance for a few days," said White House press secretary Kayleigh McEnany.
Anthony Fauci's testimony before a congressional committee started out as a gentle affair, with senators treating him as a longtime public health official, whose board carries considerable authority and experience.
Then Senator Rand Paul spoke.
As for the coronavirus, said the Republican of Kentucky, public health experts made "wrong predictions after wrong predictions after wrong predictions".
"As much as I respect you, Dr. Fauci," he continued, "I don't think you're the end of it, I don't think you're the only person who makes a decision."
Paul said that "outside New England", the virus has taken a "relatively benign course". He told Fauci that schools, closed in the US since mid-March, should be reopened as soon as possible.
Paul was expressing growing sentiment among the conservative base that Fauci does not have the best interests of the nation – or the president – in mind.
Multitudes of anti-lockdown activists in several states sang "fire Fauci", and the director demanded special protection after receiving death threats. They, like Paul, have rejected the spread of the virus beyond the current coastal hotspots, although data indicate that cases are increasing in some areas of the interior of the United States.
It is evidence of a growing political divide within the country as to how to respond to the virus and how and when to start to diminish existing mitigation efforts ordered by the state.
Dr. Fauci, for his part, responded coldly.
"I never made myself sound like the final and only voice of that," he said to Paul. "I am a scientist, doctor and public health officer."
He added that there is still a lot that is not known about the virus and warned not to be "careless" with the threat to children, given new evidence that they may not be "completely immune to the harmful effects" of the virus.
Dr. Paul, a practicing physician, was diagnosed with the coronavirus last month and received significant criticism for continuing to work in the Senate after being exposed, but before receiving his positive diagnosis.
Some states in the U.S. are beginning to lift block requests. Republican-run states such as Georgia, Oklahoma and Alaska have moved first toward relaxing restrictions.
Tuesday's hearing was Fauci's first appearance before lawmakers since President Donald Trump declared a state of national emergency in March.
The senior health advisor – who became the public face of the fight against the virus in the U.S. – was barred from testifying to a House committee led by Democrats examining the Trump administration's response to the pandemic on May 6.
How is the situation in the USA?
The United States alone has more than 1.3 million confirmed cases, according to Johns Hopkins University coronavirus tracker – almost six times more than any other country.
President Trump said the United States "prevailed" in testing people for coronavirus infections at a news conference on Monday.
But as of this week, the United States has tested only 2.75% of its population of 330 million, and no state has tested 10% of residents.
In a separate development, the White House team was ordered to wear masks when entering the west wing after two aides tested positive for the coronavirus.
Trump said he did not need to follow the directive, as he remained "away from everyone".