President Trump tweeted on Sunday that he is “immune” to the novel coronavirus and “can’t give it,” even though the White House has not released any negative test results and immunity to the virus remains poorly understood.

The tweet was quickly flagged by Twitter, which said it contained “misleading and potentially harmful misinformation” related to the coronavirus. It was the latest example of the social media giant pushing back against the president’s posts on the deadly virus, and it appeared to refer to Trump’s claim to immunity. Some recovered patients with covid-19, the disease caused by the virus, have been reinfected, and experts say many questions remain about immunity, including how long it lasts.

“A total and complete sign off from White House Doctors yesterday,” Trump said. “That means I can’t get it (immune), and can’t give it. Very nice to know!!!”

Trump’s claim came one day after his physician said he is “no longer considered a transmission risk to others,” in a memo that seemed to clear Trump to return to his normal activities a little more than a week after he announced he had tested positive for the coronavirus. Trump is expected to hold a campaign rally Monday in Florida.

Some experts said the letter provided some reassurance that Trump is no longer contagious, but they noted that there is no way to know for sure so soon after a covid-19 diagnosis. The White House has never made clear the severity of Trump’s illness, which could influence how long he should isolate.

The letter from Sean Conley said Trump had met the Centers for Disease Control and Prevention’s criteria for “safe discontinuation of isolation” and that “an assortment of diagnostic tests” found no evidence of actively replicating virus, which must be present for someone to infect others.

It did not say that Trump had tested negative for the virus, however, and its brevity left experts puzzling over what evidence had led the White House physician to conclude the president is no longer contagious.

“The honest answer is, of course, we don’t know, because they haven’t really been fully forthcoming with information either about his treatment or his clinical status,” said Arthur Reingold, who chairs the epidemiology division at the University of California at Berkeley School of Public Health.

Trump, Conley wrote, was 10 days from the onset of symptoms, had been fever-free for well over 24 hours and his symptoms had improved. That would mean he had met standards at which the CDC says people with mild to moderate cases of covid-19 can stop isolating.

But people with severe cases are advised to isolate for up to 20 days, the CDC says. Trump was hospitalized, administered supplemental oxygen and treated with the steroid dexamethasone, a drug typically used for serious cases, said Albert Ko, an infectious-disease expert at the Yale School of Public Health.

“I think the big question is whether the president had severe or he had mild, moderate disease,” Ko said. “Regardless of what the rules are, I think most physicians would want to be cautious not only about protecting the president, but protecting the people around him. That’s usually our rules of practice. Why risk it?”

Tests can provide other clues as to a person’s contagiousness, but none are foolproof, experts said.

Conley’s memo did not detail the “assortment of diagnostic” tests Trump’s health-care team has used to assess his level of illness. But it said testing throughout the president’s illness had “demonstrated decreasing viral loads that correlate with increasing cycle threshold times, as well as decreasing and now undetectable subgenomic mRNA.”

A negative PCR test, the common laboratory test that detects the virus from nasal and throat swabs, would provide a fairly clear indication that Trump is no longer infectious, so releasing a negative result would “be to his benefit,” Ko said. But the president’s medical team has not released information about any of his test results other than the first positive test on Oct. 2.

Reingold said Conley’s memo suggests that Trump tested positive but that the result indicated a low level of viral nucleic acids. “For somebody who clearly was infectious, as the president was, it’s probably good to get a couple of negative tests, a couple days apart,” Reingold said.

A positive PCR test, however, would not necessarily mean Trump is contagious, experts said. People can test positive for weeks, even months, after they’ve been infected, because the test is “just as good at picking up the remnant dead fragments of virus DNA as it is picking it up when it’s alive,” said Jeanne Marrazzo, an infectious-disease expert at the University of Alabama at Birmingham.

Trump’s team might have been using quantitative PCR testing, which is not used widely for the coronavirus but would allow them to measure the amount of virus over time, Marrazzo said. “In general, the more virus you have, the earlier you are in the course of illness and the more likely it is to be healthy, replicating alive, infectious virus,” she said.

Conley’s reference to “increasing cycle threshold times,” means the PCR test takes longer to detect the virus, which can be another sign that the level of virus is decreasing, she said. But Ko said that hasn’t been rigorously tested, and “we don’t know what the cutoff is” for a cycle length that indicates someone is no longer infectious.

Marrazzo said quantitative PCR test results typically correlate with results from the gold standard for detecting live, actively replicating virus: culturing it in a lab to see whether it infects and grows on cells. Culturing is rare, because it is time-consuming and dangerous, and even its results can vary widely and be inaccurate. Conley’s letter did not say whether this was done to determine Trump’s infectiousness.

Conley’s reference to “subgenomic mRNA” indicates Trump’s medical team has used an emerging lab test that detects actively replicating virus, experts said, and that they found none. New research suggests this test, too, may serve as a proxy for culturing.

“These are all kind of experimental diagnostic methods that kind of make sense, and I think Conley is putting it in there to bolster the argument,” Ko said. “But it doesn’t tell us that the president absolutely does not have infectious virus.”

Taken together, the references to testing and symptoms in the memo — while cryptic — are “reassuring,” Marrazzo said. But that assessment, she said, depends on believing that Trump is 10 days past the onset of symptoms and had a mild case, which the sketchy details released from the White House over the course of his illness have not made clear.

“What’s driving this very clearly is his desire to be able to be out and about and mingle,” Marrazzo said of Trump.

As for immunity, Reingold said it is too soon for a test to detect antibodies to the virus. Those typically take two or three weeks after infection to show up, he said.

Trump, who has been lagging behind Democratic presidential candidate Joe Biden in polls, is expected to tout his swift return to work and his administration’s response to the coronavirus at rallies this week in Florida, Pennsylvania and Iowa. A new Trump campaign ad, released Saturday, claims that Trump “tackled the virus head-on, as leaders should.”

“In my nearly five decades of public service, I have never publicly endorsed any political candidate,” Fauci said in a statement to CNN. “The comments attributed to me without my permission in the GOP campaign ad were taken out of context from a broad statement I made months ago about the efforts of federal public health officials.”

The ad includes a clip of Fauci speaking during an interview with conservative Fox News host Mark Levin in late March, during which Levin asked Fauci about the coordinated response of the Trump administration.

In his response, Fauci noted that he is “one of many people on a team” and spoke at length about the long hours that he and others within the administration were working to combat the pandemic.

“There’s a whole group of us that are doing that,” Fauci said.

But part of that TV ad was rebutted Sunday by Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, who said the Trump campaign used his words out of context and without his permission.“It’s every single day. So I can’t imagine that under any circumstances, that anybody could be doing more. I mean, obviously, we’re fighting a formidable enemy, this virus.”

Although Fauci did not mention Trump in his answer, the Trump campaign clip is edited to make it appear that Fauci is praising the president’s personal leadership during the crisis.

Tim Murtaugh, a Trump campaign spokesman, defended the ad on Sunday.

“The video is from a nationally broadcast television interview in which Dr. Fauci was praising the work of the Trump Administration,” he said. “The words spoken are accurate, and directly from Dr. Fauci’s mouth. As Dr. Fauci recently testified in the Senate, President Trump took the virus seriously from the beginning, acted quickly, and saved lives.”

Washingtonpost