Ohio Gov. Mike DeWine’s on-again, off-again diagnosis is troubling public health experts, who on Friday expressed fear that the prominent Republican’s apparently erroneous positive coronavirus test a day prior will become a misleading data point for those who doubt the pandemic’s severity or, in some cases, its very existence.

Meanwhile, medical researchers warned that the U.S. death toll could climb to nearly 300,000 by December — about double the current tally — in what represented one of the most dire appeals to date for people to wear masks while in public. If 95 percent of the population were to do so, according to officials with the University of Washington’s Institute for Health Metrics and Evaluation researchers, approximately 66,000 American lives could be saved.

As of Friday, more than 156,000 coronavirus-related deaths have been recorded nationwide.

Talks regarding a coronavirus relief package collapsed on Capitol Hill, and White House officials said they will recommend that President Trump move ahead without Congress to address unemployment benefits, eviction rules and student loan relief.

“The president would like us to make a deal, but unfortunately we did not make any progress today,” Treasury Secretary Steven Mnuchin said after he and White House Chief of Staff Mark Meadows met with House Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles E. Schumer (D-N.Y.). “At this point we are going to recommend to the president that over the weekend we move forward with some executive actions,” Mnuchin said.

DeWine announced Thursday that he had tested positive for the coronavirus — which scotched his plan to accompany Trump during the president’s visit to Ohio, once again a key battleground state ahead of November’s election. Nine hours later, DeWine reported that a second test had come back negative.

The governor opened his Friday coronavirus news conference by holding up a copy of the Columbus Dispatch, which carried a headline about his false positive. His wife, who was also tested, had made chicken soup while the two waited for the results of the second test, believing they would be “settled in for the long haul,” he said.

Then came the negative result, which, DeWine said, “we were obviously happy about.”

Misinformation, myths and conspiracy theories have thrived amid the pandemic, amplified by social media users with major followings. Videos painting the coronavirus as a sinister plot have themselves gone viral — online.

Trump has contributed to the climate suspicion, saying that the world-high U.S. coronavirus numbers are a function of wider testing rather than wider underlying infection. Public health experts have rejected this reasoning. He dismissed the threat early on and continues to say it will disappear, minimizing the illness even as he has rued its deadly effects. At least two personal friends of the president have died of the virus.

This week, Trump accelerated his already optimistic timeline for a potential vaccine, saying that rather than getting one into wide use by the end of the year or in early 2021, which in itself would be unprecedented speed, he now believes it could happen before the Nov. 3 election.

DeWine said Trump called him after being told the second test was negative “to see how I was getting along.”

“We had a good conversation,” the governor said.

The two men had butted heads in the spring over the Ohio Republican’s mitigation efforts, which were more stringent than those of many other Republican state executives.

Earlier Friday, DeWine sought to counter skepticism about testing and Ohio’s coronavirus statistics in light of his own experience. Ohio’s tallies, like all others, rely on test results.

“No one should take the results of this test and say, ‘Oh, none of these numbers are right,’ ” the governor said Friday morning on Columbus radio station WCOL. “There’s always a possibility for error with these tests. But what we have been using in Ohio has been the gold standard for testing.”

The governor’s office said the second round of testing used a polymerase chain reaction (PCR) test, a more accurate, more widely utilized option than the rapid antigen test from earlier in the day. But at least one epidemiologist is concerned the false positive could add to public mistrust over the severity of the pandemic.

“That’s one of my worries: People will hear this and say, ‘Well, how do we know all these cases are what they think they are? These devices don’t work and are giving a bunch of false positives,’” said Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security. “Which is largely not true.”

False results can come down to user error, she said, noting that the antigen tests can be performed in a clinic rather than a laboratory with highly trained staff. The antigen tests, which received emergency-use authorization from the FDA, are also newer.

Much of the concern over accuracy has centered on false negatives. That is because missing an infection could mean a person goes out and infects someone else. However, Nuzzo said, “false positives are not without consequences.” A person who believes they have the virus could, for instance, be unable to return to work.

During his Friday news conference, DeWine called in Peter Mohler, the chief scientific officer at Ohio State University’s Wexner Medical Center, where he took the second test, to describe the different testing methods.

Mohler likened the PCR test to a high-power telescope and the antigen test to a pair of binoculars. The antigen test is newer, less sensitive and sometimes returns false results, but it’s also faster and more accessible, he said.

“Like binoculars, you’re going to miss some stars,” Mohler said. “But the good part is, you’re going to be able to have lots of these across the field.”

DeWine plans to be tested again Saturday.

It’s not unusual for tests to show false positives. In those cases, the sensitive PCR test can detect tiny amounts of the RNA from the coronavirus. Those are known as small viral loads. PCR tests, however, might not pick up pieces of the virus on a second try, said Anne Wyllie, an associate research scientist in epidemiology at the Yale School of Public Health.

She said that in such cases, patients should take a third test, largely but not entirely for peace of mind.

Nuzzo said she worried some might seize upon the DeWine saga to question the reliability of testing or the true extent of the virus: “My fear is that unless we have very rigorous protocols for dealing with this, people may wrongly conclude that my positive test is not positive or that the covid-19 case numbers are overreported.”

Such reaction was evident in the online response to DeWine’s negative test result. Beneath a post announcing the news on the governor’s Facebook page, comments piled up claiming that testing is unreliable and the shutdown is based on faulty data.

“How many false positives have we added to our count?” asked one woman. “Unbelievable. People’s lives and livelihoods are being taken away on possible false testing results.”

A Republican state lawmaker, Rep. Nino Vitale, added to the uproar. In a post on his Facebook page, he cast testing as flawed and the pandemic as overblown.

“If DeWine can test positive and then negative inside of a few hours, how many thousands of tests are false? I say, ALL OF THEM,” he wrote. He discouraged testing, writing that it’s “meaningless” and “the government and media are using it to inflate the numbers and inflate the fear.”

Others called attention to the governor’s access to testing that can be in short supply for most people.

Steven Mufson contributed to this report.