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After an unprecedented surge in new cases of COVID-19 over the past several weeks, on November 19 the Centers for Disease Control and Prevention urged Americans not to travel for Thanksgiving. At the briefing—the agency’s first in months—officials emphasized that the safest way to celebrate is with your immediate household. The CDC does not endorse taking a COVID-19 test before attending a Thanksgiving gathering, reported The New York Times.

“I feel like Thanksgiving is screwed, but I don’t know that there’s a way to test yourself into safety,” says Yukari Manabe, a professor of medicine at the Johns Hopkins University School of Medicine, who recently discussed the need for more and better testing for COVID-19 in the Journal of the American Medical Association (JAMA). “This is a year where if we start mixing, we’re just going to accelerate the infection, and we’re already at unprecedented levels nationwide with [hospital] bed capacity quickly dwindling.”

Even a negative test result cannot make a Thanksgiving gathering completely risk-free. And in any situation, testing is no substitute for masks, social distancing, and quarantining experts say. “It’s one way to decrease the risk, but it’s not ironclad and still should be coupled with other common sense measures,” says Amesh Adalja, an infectious disease physician and researcher at the Johns Hopkins University Center for Health Security. “You can’t just rely on the test.”

Here’s why diagnostic tests aren’t a guarantee of safety, and what a COVID-19 test can—and cannot—tell you.

A moment in time

If you were exposed to the novel coronavirus today and took a diagnostic test tomorrow, the test would not be able to detect the infection, Adalja says. “It’s important to remember that a test is one moment in time,” he says.

It takes several days for the virus to accumulate to levels that a test can pick up. “If you were someone who did not have a high viral load on Thursday…that viral load may have reached a point where the test will pick it up on Friday,” says Neha Nanda, medical director of infection prevention and antimicrobial stewardship at Keck Medicine of USC. A good time to test is around five days after you think you were exposed to the virus, she says. While the incubation period for SARS-CoV-2 can be anywhere from 2 to 14 days, the average time that people start developing symptoms is 5 days after exposure.

And remember: Don’t skimp on quarantining before the test. People can become contagious several days before they develop symptoms of COVID-19, and may in fact be most likely to transmit the virus during this time. People who never develop symptoms can also be contagious.

There’s also the possibility that you will become infected between the time of your test and seeing other people. This is why it’s essential to stay vigilant even after receiving a negative test, says Valerie Fitzhugh, the interim chair of the Department of Pathology at the Rutgers New Jersey Medical School. “If I am tested on Monday and a few days later, I get a negative result, all it tells me is that I was negative on Monday,” she said in an email. “If you go out into the world, you don’t socially distance, or you don’t wear a mask, you could become positive after your negative test.”

One example of the limits of relying on diagnostic tests at the expense of other precautions is the Rose Garden ceremony in which President Trump presented Amy Coney Barrett as his nominee to the Supreme Court in September, Adalja says. The White House said that all guests had tested negative for COVID-19 before the event. During the ceremony, many people didn’t wear masks or practice social distancing. After a cluster of attendees later tested positive for the novel virus, the ceremony was recognized as a potential superspreader event.

Types of tests

Another consideration is the kind of COVID-19 test that you’re taking. Molecular tests, such as PCR tests, detect genetic material from the virus. Antigen tests, on the other hand, look for proteins on the surface of the virus. Antibody tests detect disease-fighting antibodies that your immune system has produced days or weeks after encountering the novel coronavirus, SARS-CoV-2. Antibody tests can reveal if you’ve had COVID-19 in the past, whereas molecular and antigen tests can determine whether you currently have an active infection.

Antigen tests typically return results more quickly; some rapid tests have a wait time of only 15 minutes, while it can take days to hear back after taking a molecular test. These tests are less likely to miss an active infection, though. “A PCR test is meant to be exquisitely sensitive to pick up minute amounts of…viral genetic material,” Adalja says. However, some people may test positive for weeks after being diagnosed with COVID-19 because they still carry residual, non-infectious bits of the virus in their bodies.

“Antigen tests are pretty good at finding people with highly transmissible disease, but…if you’re on the quick upswing of asymptomatic disease towards symptomatic infection you may go from negative to positive on an antigen test even within 12 to 14 hours,” Manabe says. “If you have symptoms and you’re worried that you were exposed to COVID and that you have COVID, both test types are going to do a pretty good job.”

The time for testing

In a less imperfect world, you’d time your COVID-19 test as close as possible to an event where you will see people outside your household. “However, many of the testing sites in this country are inundated with people getting tested and there are long lines and long turnaround times, so you may not get your result back in time anyway,” Adalja says.

Some places, such as South Korea and Taiwan, have made substantial investments in COVID-19 testing since the beginning of the pandemic. Unfortunately, the testing capacity of the United States continues to lag behind. Ideally, “yes, you would get tested multiple times going up to and even on the day-of, because you would have a rapid result back an hour before you needed to go to the Thanksgiving dinner,” Adalja says. “That’s how we would do it if we lived in a country that really had prioritized testing.”

As it is, Manabe is concerned about the increased demand for COVID-19 tests ahead of the holiday. “We’re going to overrun the testing capacity of America, doing tests just so people can go places for Thanksgiving,” she says. Part of the problem is that we did not have enough tests to get a handle on the virus over the summer, when the virus was less prevalent across the country and tracing the contacts of those who tested positive would have been easier. “Tests are going to be more effective in helping to keep the pandemic under control when we have low community transmission,” Manabe says. “Right now, we’re just playing catchup.”

If you will be attending a Thanksgiving dinner, ideally you’d have planned to stay home for the two weeks before the event. If you went through testing, after going to get the test itself, you should quarantine until Thanksgiving, Fitzhugh says. And if you must travel, find out what testing and quarantine requirements the state you’re bound for has for visitors.

“In the end it’s all going to boil down to risk tolerance…because you’re not going to be able to get the risk of your Thanksgiving dinner to zero, and the more people you have at your event, the more [different] households, the more places they’re coming from, the more likely the virus is going to be sitting around your table as well,” Adalja says. “So it’s really important even if you got tested to still think about the fact that you need to be very mindful about people’s contact with each other, try to do things outdoors, try to open windows, have people washing their hands, [and] wearing face coverings as much as they can when they’re not eating or drinking.”

It’s also important to stay home if you start to feel sick, Nanda says. “A negative test is not 100 percent foolproof…it’s not your ticket to freedom from COVID,” she says. “The minute you decide to do something more than a virtual setup you are putting yourself at risk, period.”

Testing is only one part of our approach to tackling COVID-19, Fitzhugh says. “We must continue to wear masks, wash our hands, and socially distance to curb the spread of this disease,” she said. “If that means we don’t get to have large Thanksgiving, Christmas, and New Year’s gatherings, it is worth it to be able to have them next year, to get our students back into schools, and to give our healthcare workers a well-deserved break.”

Like Fitzhugh, Manabe advises staying in your own social bubble for Thanksgiving. “If we can just get to levels where community transmission is low enough that we’re not overwhelming our hospitals, then we’ll make it to a time when there’s going to be enough vaccines available that we can start again,” she says. At that point, it will be easier to use testing and contact tracing to keep a lid on the virus. Manabe also sees several improvements to COVID-19 testing on the horizon.

On November 17, the Food and Drug Administration authorized the first at-home COVID-19 test that can return results within an hour. The test, which requires a prescription, will become widely available in spring 2021. And on November 20, the National Institutes of Health announced that it will be expanding its Rapid Acceleration of Diagnostics Underserved Populations program, which seeks to make COVID-19 testing more accessible for those disproportionately struck by the virus.

“We want turnaround times that are under 24 hours in order to be able to interrupt transmission,” Manabe says. “By the new year we’re going to start to see huge numbers of tests available so that symptomatic people can get a test and they can get a result relatively quickly, and that will be good for everyone.”

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