Here’s why you can be contagious but still test negative

If you think a negative test result means you don’t have coronavirus, you could be wrong.

a man wearing a hat: A man gets swabbed by CNA Keila Kelley at a free COVID-19 testing site on Front Street in Reading, PA outside FirstEnergy Stadium on Tuesday morning October 13, 2020. The site will be there for 5 days and was setup in response to an increase in cases in Berks County. (Photo by Ben Hasty/MediaNews Group/Reading Eagle via Getty Images)

© Ben Hasty/MediaNews Group/Reading Eagle/Getty Images
A man gets swabbed by CNA Keila Kelley at a free COVID-19 testing site on Front Street in Reading, PA outside FirstEnergy Stadium on Tuesday morning October 13, 2020. The site will be there for 5 days and was setup in response to an increase in cases in Berks County. (Photo by Ben Hasty/MediaNews Group/Reading Eagle via Getty Images)

It can take days before a new infection shows up on a Covid-19 test.

“We know that the incubation period for Covid-19 is up to 14 days. And before that, you can be testing negative, and have no symptoms,” emergency medicine physician Dr. Leana Wen told CNN. “But you could actually be harboring the virus and be able to transmit it to others.”

So if you want to get tested as a precaution before seeing friends or family, here’s what you need to know:

If I got infected yesterday, would a test today pick that up?

Probably not. A study in the medical journal Annals of Internal Medicine examined false-negative test results of people who actually had Covid-19.

The study estimated that during four days of infection before symptoms typically started, the probability of getting an incorrect/negative test result on Day 1 was 100%.

On the day people started showing symptoms, the average false-negative rate had dropped to 38%, according to the study. Three days after symptoms started, the false-negative rate dropped to 20%.

“The virus just takes time to replicate in the body to detectable levels,” said Justin Lessler, a senior author of the study and associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

“You can get infected by just a few viral particles, but these will not be detectable until they have time to replicate to adequate levels to be detected,” he told CNN by email.

So how many days should a person wait after possible exposure to get tested?

“There is no hard and fast rule, but the evidence suggests getting a test before the third day after exposure is not of much use,” Lessler said.

Could I be contagious while testing negative?

Absolutely. “People sort of feel like if you test (negative), you’re out of the woods. And you’re kind of not,” said Dr. Rochelle Walensky, chief of the infectious diseases division at Massachusetts General Hospital.

For people who get sick with Covid-19, symptoms can take up to two weeks to appear, but the average time is about five days, Walensky said.

“It’s generally thought that you’re most infectious the two days before that day and the two days after that,” she said.

One reason why this virus spreads so easily is because people can be infectious without any symptoms. The US Centers for Disease Control and Prevention estimates 40% of infections are asymptomatic, and 50% of transmissions happen before symptoms begin.

“It’s been among the biggest Achilles’ heels

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Trump isn’t contagious now. But his administration will still get people sick.

While Trump’s speedy return to large public events was alarming, it’s unlikely he exposed many of his supporters at his first rallies after his diagnosis this month to the virus. The distance the Secret Service tends to require between presidents and big crowds alone should have protected most of his voters. What’s more disturbing to epidemiologists like me is that Trump continues to act in ways — and model behavior — that run directly counter to best practices for controlling infections. Even after becoming the world’s most famous covid-19 patient, Trump demonstrates an utter lack of seriousness or willingness to manage the virus.

The president’s diagnosis last month was not a surprise to infectious-disease experts, as the White House approach to prevention had been focused on testing, which is a secondary form of prevention. Use of testing as the only strategy is highly imperfect, especially with rapid tests, as what we gain in speed often costs us accuracy. Data on accuracy is based on samples from sick or exposed people, not necessarily on a healthy population using this frequently as a mechanism to avoid masking and distancing. The administration’s lackluster adherence to masking, distancing and overall risk reduction measures meant an outbreak centered on the White House was not particularly shocking, but it was nonetheless disappointing.

Trump held his first post-hospital rally in Sanford, Fla., last Monday. The White House still hadn’t been particularly transparent about his illness or the timeline for infection and contagiousness, but Sean P. Conley, Trump’s physician, released a memo stating vaguely that multiple tests had shown the president was negative for the disease. That memo, emphasizing the use of rapid testing, raised concerns for several reasons. The Abbott rapid tests being used by the White House have been shown to have higher rates of false negatives. But the public still didn’t know if the president experienced severe disease, which would require a longer isolation period. From an epidemiological and infection prevention perspective, this knowledge is critical: Understanding when his symptoms began and when he tested negative and then positive helps us gauge when he was probably infectious (guidance from the Centers for Disease Control and Prevention recommends we start contact tracing on the two days before symptom onset or a positive test). These pieces not only allow us to identify potential cases moving forward, but they would also shed light on whether Trump was infectious during the presidential debate with former vice president Joe Biden last month and at events just before his diagnosis. As the White House has opted not to engage in contact tracing, this has left much to crowdsourcing in an attempt to get a handle on what many are calling a superspreader event stemming from the Sept. 26 Supreme Court nomination event for Amy Coney Barrett in the Rose Garden.

Trump still hasn’t really disclosed much about his test results that week, telling NBC News’s Savannah Guthrie on Thursday night that “possibly I did, possibly I didn’t” take a coronavirus test

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