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:
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 of this. And it’s been among the biggest challenges and the unexpected things because (with) its cousins, this is not true,” Walensky said.
“The reason we were able to control the SARS outbreak so quickly — although there were obviously a lot of deaths there — but it didn’t turn into a pandemic is because people weren’t shedding (the virus) until they got symptoms.”
Do different kinds of Covid-19 tests matter?
When it comes to diagnostic tests — those that detect whether you have an active coronavirus infection — there are two main types:
“This test is typically highly accurate and usually does not need to be repeated,” the FDA says.
But the downside to molecular testing is that results can take a while — anywhere from the same day to one week after testing.
But for infected people who don’t get symptoms, the accuracy rates are less clear, she said. “Molecular test performance studies on asymptomatic people are very limited.”
Antigen tests don’t look for the virus’ genetic material, like molecular tests do. Instead, they look for specific proteins on the surface of the virus.
The good news is you can get antigen test results in less than an hour. The bad news is you’re more likely to get a false negative with a rapid antigen test.
“Positive results are usually highly accurate but negative results may need to be confirmed with a molecular test,” the FDA says.
“Antigen tests are more likely to miss an active coronavirus infection compared to molecular tests.”
This could help explain some of the recent spread of Covid-19 linked to the White House.
Regardless of which type of diagnostic test you use, you’re generally more likely to get a false negative than a false positive.
“If a molecular test is positive, it’s an accurate reflection of a person being infected,” MacDonald said. “If it’s negative, it’s less reliable that the person is indeed negative. The same is true of the antigen tests.”
Can I test myself at home?
But taking any kind of Covid-19 test too early might miss an infection, said Dr. Frank Ong, chief medical and scientific officer at Everlywell.
“As testing capacity has continued to increase, more and more asymptomatic or mildly symptomatic individuals have undergone testing, most of whom likely have lower viral loads in their clinical samples,” he said.
“My best advice is to treat yourself as if you are likely infected after suspected exposure. Quarantine, wear a mask to cover both nose and mouth, and practice good hygiene practices. If you want to take extra precautions or you believe you may have been exposed, get tested for the virus using an FDA-authorized test for Covid-19.”
So what should I do if I want to see friends or relatives?
If you insist on seeing loved ones for Thanksgiving, self-quarantining for 14 days beforehand is probably your safest bet, Walensky said.
“A 14-day quarantine — with a real quarantine — if you do that properly, you don’t need a test,” she said. “That’s probably the cleanest way to do it.”
To be clear: Quarantining means staying home. It does not mean you can run errands.
“‘Grocery store’ and ‘quarantine’ don’t belong in the same sentence,” Walensky said.
Adding testing to your list of precautions “is a helpful thing, but only if it’s done properly,” she said. “And if you don’t know exactly when or how to use it, then you very well could be setting yourself up for a false sense of security.”
Lessler agreed that quarantining is best, and testing must be done intelligently.
“If you are visiting an elderly family member and have a reasonable risk of having been exposed, there is no substitute for 14 days of quarantine,” Lessler said.
“At the very least I would wait 10 days and have a negative test. If you are visiting a younger, healthy family member and have little chance of being exposed before or during travel, then 5 or 7 days (of quarantine) plus a negative test is probably plenty of risk reduction, though no guarantee of safety. Either way, you should definitely remain in quarantine while awaiting test results and make sure everyone you are getting together with is on the same page about the plan for controlling infection risk.”
There have already been cases of coronavirus spread within families just days after a person tested negative, said Dr. Michael St. Louis, a member of the CDC’s Community Guidance Team.
He said everyone must remember to treat family from different households the same way you would treat unrelated friends or work colleagues during this pandemic.
Of course, the best way to help ensure the safety of all your loved ones is to celebrate the holidays remotely.
“I have three kids. … And my parents are not going to be joining us this year,” Walensky said.
“It’s just awful. But what I really hold out for is that my parents are pretty healthy, and I would never forgive myself if I put them in harm’s way. And I’m just looking forward to a 2021 when we can be together.”
Walensky said the small sacrifices made this Thanksgiving will help ensure everyone will be healthy enough to sit at the table next year.
“At least they’ll be there next year, whereas irresponsible behavior now might mean they won’t be here later,” she said.
“Let’s do this so that we can have a much better shot of being around the table together, healthy, in 2021.”
CNN’s Lauren Mascarenhas contributed to this report.