- President Trump tested positive for the coronavirus early Friday morning.
- The virus' average incubation period — the time between when a person gets infected and when they test positive or show symptoms — is five days.
- Research has found that infected people shed the most coronavirus the day they start showing symptoms and the day before.
- Here's what that means for Trump's contacts, including Joe Biden.
- Visit Business Insider's homepage for more stories.
After President Donald Trump announced that he and Melania Trump had been diagnosed with the coronavirus, public-health experts started doing some simple math.
The president tweeted about his positive test result at 1 a.m. ET on Friday. Researchers know that on average, it takes five days for coronavirus symptoms to show up after a person gets infected. That's also how long it might take for a test to come back positive after exposure. So the president likely got exposed sometime in the last week.
Several studies have also found that that coronavirus patients are often contagious before showing symptoms — in fact, the average patient is most infectious the day they start showing symptoms and the day or two prior.
Here's what the research shows about the virus' incubation period, when a person is most likely to transmit it, and what that means for everyone who recently interacted with Trump.
The virus' incubation period is between 2 and 14 days
The coronavirus' incubation period — the length of time between when a person gets infected and when they first show symptoms or test positive — is between two and 14 days, according to the Centers for Disease Control and Prevention. Similarly, a March study of coronavirus patients in China earlier this year reported that the incubation period for 97.5% of infected people was less than 12 days.
But on average, it takes four to five days for symptoms like fatigue, cough, and fever to start.
Trump developed symptoms, including a cough, fever, and lethargy on Friday, the New York Times reported. In the afternoon, he was transferred to Walter Reed National Military Medical Center for testing. He will remain at the hospital for a few days, according to the Associated Press.
This timeline suggests Trump may have gotten infected last weekend.
However, because it takes a couple of days for people to start shedding viral particles after infection, Trump most likely did not spread the virus to any attendees at Judge Amy Coney Barrett's Supreme Court nomination on Saturday.
"It's safe to say that it would be exceedingly rare for anyone to transmit the virus earlier than two days post exposure; however, at some point after that, the risk would begin to rise significantly," an article on the MIT Medical website says.
According to Monica Gandhi, an infectious-disease expert at the University of California, San Francisco, anyone who has recently been around Trump, Melania, or White House adviser Hope Hicks (who tested positive Thursday) could test positive in the days ahead, even if they've recently had a negative test.
"This entire week would be a risky period," Gandhi told Business Insider.
Trump was probably infectious a day before his symptoms started
Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security, told Business Insider that the president was infectious "probably about a day before symptoms started."
How contagious a given patient is has to do with their viral load: how many viral particles they're carrying and shedding into the environment. One April study showed that patients had the highest viral load in their throat swabs at the time their symptoms started.
"Models show the highest infectious period, the period at which you're spewing the most virus, is your onset of symptoms and possibly the day before," Gandhi said.
That's why pre-symptomatic coronavirus transmission is so common. And it's why contact tracers usually track people's contacts back "about 24 hours before symptom onset," Adalja said.
He added that an infected person's period of peak viral load can persist into "the very first days of symptoms" as well. That means the 100 or so people Trump came into contact with at his Thursday fundraiser in Bedminster, New Jersey, may have been exposed to the president at his most infectious time.
Trump's campaign sent all fundraiser attendees an email on Friday urging them to contact a medical provider if they develop COVID-19 symptoms.
The timeline could shift, however, if Trump turns out to have been feeling mild symptoms before Friday. Bloomberg reported that his aides suspected he wasn't feeling 100% as early as Wednesday.
If that's true, the president might have been at his most infectious point on Tuesday, when he was debating former Vice President Joe Biden in Cleveland, Ohio.
Even though Trump's COVID-19 test came back negative on Tuesday, Adalja said, a person "could be infectious and test negative."
Trump's contacts who test negative Friday aren't 'in the clear'
Biden and his wife, Jill, tested negative for the coronavirus on Friday, after which he announced he'd be resuming cross-country campaigning.
But health experts caution that the negative result doesn't mean Biden can rest easy yet.
"Reminder that testing COVID-19 negative today does not mean that those exposed to POTUS/FLOTUS/Hope Hicks are in the clear," Anne Rimoin, an epidemiologist at University of California, Los Angeles, tweeted on Friday.
The virus' four- to five-day average incubation period means that if Biden was infected at the debate, he would most likely not start to show symptoms or test positive until this weekend.
Although Biden and Trump stood about 10 feet apart onstage, the event was held inside and neither man wore a mask as they vigorously debated. Studies have shown that talking loudly and forcefully exhaling can lead a person to spew viral droplets farther than 6 feet.
Gandhi said that is not unreasonable to be concerned about Biden's potential exposure.
"The concerning part is that Trump yelled straight for 90 minutes," she said, adding, "I would advise Biden, certainly, to get tested."
Morgan McFall-Johnsen contributed reporting to this story.
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