Trump returned to the Oval Office. He may still be contagious.

There’s a lot we still don’t know about President Trump’s Covid-19 case, starting with the basics: Is he having trouble breathing? Is he experiencing side effects, such as delirium or confusion, from the multiple experimental drugs for Covid-19 he’s taking? When was his last negative coronavirus test?

There’s another burning question about Trump’s health that matters more for the people around him, including his family members and staff at the White House who haven’t yet tested positive: Is the president still contagious?

Patients who have been diagnosed with Covid-19 are supposed to — per Centers of Disease Control and Prevention guidelines — isolate for at least 10 days since they first noticed they were sick or tested positive and until their symptoms have improved, including no fevers for at least 24 hours.

In cases like Trump’s that required hospitalization, that isolation period can last even longer. “For patients who had severe or critical illness, we wait up to 20 days to state they are noninfectious,” said Sigal Yawetz, an attending physician at Brigham and Women’s Hospital in Boston who has been caring for Covid-19 patients.

Doctors rely on clinical symptoms because there’s no clinically validated way to check if a patient is still contagious — even when the patient is the president.

But we don’t know exactly when Trump became symptomatic — one of the many basic facts about his case that’s been muddled by confusing reports. If we go by what his doctors and colleagues have suggested, he started experiencing symptoms on Thursday, a day before entering hospital.

That means Trump should probably still be isolating for at least two more days — perhaps longer.

Instead, he rode in a car with others on day four of his symptoms, and on day five, took off his mask as he entered the White House. As of Wednesday, he’d returned to the Oval Office to work — necessitating those around him to don N95 masks, gowns, and gloves.

Safely donning and doffing personal protective equipment requires training and can be challenging for even highly experienced clinicians. Considering the White House’s colossal failure at basic coronavirus prevention measures — such as social distancing, mask-wearing, and contact tracing — it’s hard to imagine they’ll successfully turn the residence and Oval Office into a model of infection control.

But if the White House wants to prevent more coronavirus cases, they’ll have to for this simple fact: Trump may still be contagious.

There’s no clinically validated way to test for Covid-19 contagion

The best research we have so far on Covid-19 transmissibility suggests that people can shed infectious virus for up to nine days after getting sick. In general, they are most contagious two days before symptom onset, and in the first five days after, as the graph in the tweet below, from this Oxford University Press study, shows.

But these are averages — and they hide a lot of the variability among patients. Some people are infectious for a shorter period, while others shed virus for up to 20 days after symptom onset.

That’s why doctors and public health officials ask patients to isolate for at least 10 days after their symptoms appear, or even longer in some cases.

As for Trump, “potentially he passed through the most infectious stage already,” said University of St. Andrew’s virologist Muge Cevik. It’s also possible the medications he’s on, including the antiviral remdesivir, brought down his viral load quickly. “But he may be still contagious depending on his symptom onset.”

Doctors rely on the date symptoms appeared, or the first positive test in cases where people don’t notice symptoms, because there’s actually no proven way to show that a person is no longer infectious. The diagnostics available to check for infection — PCR tests — “can’t distinguish between intact, infectious virus and degraded viral genetic material,” Ilan Schwartz, an infectious disease doctor and professor at the University of Alberta, told Vox.

Doctors could use something called the CT, or cycle threshold, that’s part of the PCR test. It’s a measure of a person’s viral load, and if the number gets higher over time, the viral load in the sample is getting smaller. But the link between a person’s CT number and their transmissibility is still not fully understood, said University of Michigan infectious disease doctor Kevin Gregg.

“There are not completely established guidelines for [using CT values],” he added. So it’s not something doctors typically rely on to determine how long a patient should isolate.

That’s why — as a precaution — “[Trump] should be isolating,” Gregg said, “so as not to infect members of his family, the staff of the White House, and other people.”

“We don’t say, ‘It’s been 10 days, you can take off your mask’”

There is one other, more advanced, way to check on a person’s contagiousness: cell culturing. Scientists used the technique in many of the studies that informed the quarantine period. It involves infecting animal cells in Petri dishes with the SARS-CoV-2 virus, waiting for them to incubate, and seeing whether the virus can still grow in them.

One of Trump’s physicians, Jason Blaylock of Walter Reed National Military Medical Center, mentioned the president’s medical team was working with “various laboratories in the area, state of the art facilities … on obtaining advanced diagnostic testing to really inform the White House medical team of both the status of the president as well as his ability to transmit virus to others.”

He didn’t specify, but perhaps cell culturing was what he was referring to. Yet even that isn’t foolproof for use in patients. “There is a big difference between the virus being culturable in a Petri dish full of cells derived from a monkey’s kidney and being capable of infecting the respiratory epithelium of living people,” Schwartz said. “This test has absolutely not been validated for ruling out SARS-CoV-2 infectiousness in people.”

What’s more, it takes several days to grow the virus and get a reliable result. “So I don’t think they’d have that information yet,” said Gregg.

Even after isolation, patients are still advised to wear a face mask. “If someone had Covid, after the contagious period is over, it doesn’t mean you go out and do things as usual,” said Preeti Malani, an editor at JAMA and infectious disease doctor who treats Covid-19 patients. “We don’t say, ‘It’s been 10 days, you can take off your mask.’ We continue to treat people as if they might be infectious because the truth is we don’t know.”

Instead of mask-wearing, the White House has relied heavily on testing to prevent outbreaks. That, too, is problematic. “Many White House officials have been feeling comfortable after receiving a negative test,” Cevik said, “but maybe they don’t necessarily understand or haven’t been told the negative test doesn’t give you 100 percent clearance.”

As Vox’s Brian Resnick reports, coronavirus tests can fail for a multiplicity of reasons — including improper swabbing or taking the test before someone has enough virus to test positive. This means anyone who encountered Trump, who wasn’t wearing full personal protective equipment while Trump is infectious, could now be carrying the virus, even if they’ve tested negative. According to guidelines, those people should be quarantining for 14 days, too.

If the White House wants to prevent more coronavirus infections, they’ll have to change course. “Testing on its own is not going to prevent onward transmission,” Cevik said. But isolating — in addition to masks, contact tracing, social distancing, and testing — can.

Eliza Barclay contributed reporting to this story.

Correction, October 7: This article originally misattributed a quotation by Ilan Schwartz on the limits of advanced Covid-19 diagnostics to Sigal Yawetz.

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