On Thanksgiving, don’t trust a negative covid-19 test

(CNN) — If you think that a negative COVID-19 test result means you don’t have the virus, you could be wrong.

It can take days before a new infection shows up on the coronavirus test.

“We know that the incubation period for covid-19 is up to 14 days. And before that, you can test negative and have no symptoms, ”said emergency medicine doctor Dr. Leana Wen.

“But you could actually be harboring the virus and be able to pass it on to other people,” he explained.

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

If I was infected yesterday, would a test detect the virus today?

Probably not. A study in the medical journal Annals of Internal Medicine examined false negatives in tests of people who did indeed have COVID-19.

The study estimated that for four days of infection before symptoms commonly appeared, the probability of getting an incorrect / negative test result on day 1 was 100%.

On the day that people began to show symptoms, the average false negative rate dropped to 38%, according to the study. Three days after symptoms began, the false negative rate dropped to 20%.

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

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

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

“There is no definitive rule, but the evidence suggests that getting tested before the third day after exposure is not very helpful,” Lessler said.

Can I be contagious if the test is negative?

Of course. “People feel that if the test (is negative) they are out of danger. And it’s not like that, ”said Dr. Rochelle Walensky, chief of the division of infectious diseases 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.

“In general, it is thought that you are most contagious the two days before that day and the two days after,” he said.

One of the reasons this virus spreads so easily is because people can spread it without any symptoms. The US Centers for Disease Control and Prevention (CDC) estimate that 40% of infections are asymptomatic and that 50% of transmissions occur before symptoms begin.

“It was one of the biggest Achilles heels of this. And he has been between the biggest challenges and the unexpected things because (with) his cousins ​​this is not true, ”said Walensky.

“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 didn’t spread (the virus) until they had symptoms.

Do different covid-19 tests matter?

There are two main types of diagnostic tests that try to detect if you have an active coronavirus infection:

The molecular tests, like PCR tests, look for the genetic material of the virus. Most of these tests are done with nasal or throat swabs, although some can be done with saliva, according to the US Food and Drug Administration (FDA).

“This test is usually very accurate and usually does not need to be repeated,” says the FDA.

However, the downside to molecular testing is that results can take a while. In fact, they can give them to you from the same day to a week after you took the test.

“For people showing symptoms, studies so far show that the precision of molecular testing in finding a positive case increases every day after exposure,” said Pia MacDonald, infectious disease epidemiologist at the nonprofit research institute RTI International.

However, for infected people who do not have symptoms, the accuracy rates are less clear, he said. “Studies of the performance of molecular tests in asymptomatic people are very limited,” he explained.

Differences in antigen testing

The antigen testing these are often referred to as rapid tests (although some molecular tests are also rapid). Antigen tests are not antibody tests, they tell you if you have had the virus before and you have already developed antibodies against the infection.

Antigen tests do not look for the genetic material of the virus the way molecular tests do. Instead, they look for specific proteins on the surface of the virus.

The good news is that you can get your antigen test results in less than an hour. The bad news is that you are more likely to get a false negative with a rapid antigen test.

“Positive results are usually very accurate, but negative results may need to be confirmed with a molecular test,” says the FDA.

“Antigen tests are more likely to miss an active coronavirus infection compared to molecular tests,” he adds.

This could help explain some of the recent spread of covid-19 linked to the White House.

While staff members close to President Donald Trump and Vice President Mike Pence are tested frequently, White House staff often use rapid antigen tests, which generally have a higher false-negative rate than the tests. molecular.

Regardless of the type of diagnostic test you use, you are generally more likely to get a false negative than a false positive.

“If a molecular test is positive, it is an accurate reflection of an infected person,” MacDonald said.

If it is negative, it is less certain that the person is really negative. The same goes for antigen testing, ”he explained.

Can I get tested at home?

Yes. There are some home-made test kits available, such as the Everlywell Covid-19 Molecular Test. Users take their own samples with nasal swabs and mail them to a laboratory, which will send the results digitally within 24 to 48 hours after receiving the samples.

But getting any kind of COVID-19 test too early could miss an infection, said Dr. Frank Ong, Everlywell’s chief medical and scientific officer.

“As test capacity has continued to increase, more and more asymptomatic or mildly symptomatic people have been tested. Most of them probably have lower viral loads in their clinical samples, “he said.

What should I do if I want to see friends or family on Thanksgiving to minimize my risk from covid-19?

If you insist on seeing someone who doesn’t live with you on Thanksgiving, it’s best to quarantine yourself 14 days in advance, Walensky said.

“If you do it correctly, you don’t need proof,” he said. “That’s probably the best way to do it,” he added.

To be clear: self-quarantine means staying home. It doesn’t mean you can go out to run errands.

“‘Grocery store’ and ‘quarantine’ don’t go in the same sentence,” said Walensky.

Lessler agreed that quarantine is for the best and that any testing should be done wisely.

“If you are visiting an older relative and you have a reasonable risk of being exposed, there is no substitute for the 14-day quarantine,” Lessler said.

“I would at least wait 10 days (of quarantine) and have a negative test,” he said as families prepare to celebrate Thanksgiving amid the covid-19 pandemic.

“If you are visiting a younger, healthier relative and you have little chance of being exposed before or during the trip, then 5 or 7 days (of quarantine) plus a negative test is probably a big risk reduction, although there is no guarantee of security “, explained

What should I do after I test negative for covid-19 to celebrate Thanksgiving?

It is important to do a strict quarantine not only before the COVID-19 test, but also afterwards.

“You definitely want to stay in quarantine while you wait for the test results and make sure everyone you meet is on the same page about the plan to control the risk of infection,” Lessler said.

There have already been cases of the coronavirus spreading within families within days of a person testing negative, said Dr. Michael St. Louis, a member of the CDC Community Counseling Team.

St. Louis said everyone should remember to treat family members living in different homes the same way they would treat friends or co-workers during the pandemic.

What is the safest way to celebrate Thanksgiving in a covid-19 pandemic?

The best way to help everyone stay healthy and prevent COVID-19 infections is to celebrate Thanksgiving remotely.

“I have three children (…) And my parents will not join us this year,” said Walensky.

It’s just horrible. But what I really maintain is that my parents are quite healthy, and I would never forgive myself if I put them in danger. And I’m looking forward to 2021 when we can be together, “he added.

The CDC suggests celebrating with loved ones virtually. You can also prepare traditional Thanksgiving dishes and deliver them “in a non-contact way” to family, neighbors, or those who may feel lonely.

Walensky said the small sacrifices made this Thanksgiving in the COVID-19 pandemic will help ensure everyone is healthy enough to sit at the table next year.

“At least they will be there next year, whereas irresponsible behavior now could mean they won’t be here later,” he said.

“Let’s do this so we can have a better chance of being healthy around the table together in 2021,” he added.

CNN’s Lauren Mascarenha contributed to this report.