Oxford vaccine chief “optimistic” that trial results could be reached by end of 2020


A nurse hands a sample to a medical assistant at a Covid-19 testing site in Boston on October 22.
A nurse hands a sample to a medical assistant at a Covid-19 testing site in Boston on October 22. Jessica Rinaldi/The Boston Globe

If you think a negative test result means you don’t have coronavirus, you could be wrong — it can take days before a new infection shows up on a Covid-19 test. It is possible to be harboring the virus, but be testing negative and have no symptoms in the early stages of infection.

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

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, in an email.

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 added.

There are two main types of diagnostic tests that detect active coronavirus infections:

Molecular tests, such as PCR tests, look for the virus’ genetic material. Most of these tests are performed with nasal swabs or throat swabs, though some can be done using saliva, the US Food and Drug Administration says. This test is typically “highly accurate” and does not need to be repeated, according to the FDA.

Antigen tests, often known as rapid tests (though some molecular tests are rapid, too), look for specific proteins on the surface of the virus rather than the virus’ genetic material. 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.

If you insist on seeing loved ones for the holidays, self-isolating 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, adding that isolating properly means you cannot run errands or go to the grocery store. “That’s probably the cleanest way to do it.”

Adding a coronavirus test to your list of precautions “is a helpful thing, but only if it’s done properly,” Walensky 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.”

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Don't get a false sense of security with Covid-19 testing. Here's why you can test negative but still be infected and contagious



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