Just because you test negative for COVID-19 in the days prior to your Thanksgiving celebration doesn’t mean you are clear of coronavirus infection, experts say.
That’s why health professionals strongly suggest saving holiday celebrations for immediate household members only to prevent further viral spread.
The Centers for Disease Control and Prevention said Thursday that people should not travel for Thanksgiving, but rather celebrate with people you live with. The announcement comes as more than one million COVID-19 cases were reported in the U.S. in just one week.
The mix-up in results comes down to the timing of your test versus the timing of your exposure to the virus, as well as the type of test you take.
“Simply having a test at one point in time might mean that at that exact moment, you may not have a positive test, but in a few hours, a couple of days, you might turn positive,” Dr. Vanessa Walker, medical director of the Sutter Health Valley Area electronic ICU in California, told KCRA. “A negative test is not something that I feel good about when I want to expose myself to other people.”
There are two types of tests: PCR tests and rapid antigen tests.
PCR tests are the main diagnostic tests used during the pandemic, and they look for the coronavirus’ genetic material. These tests are expensive, highly sensitive and can take hours to produce results, experts say.
Meanwhile, antigen tests look for pieces of proteins that belong to the coronavirus. These tests are cheaper and faster, usually producing results within an hour.
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While both tests can tell if someone is currently infected, antigen tests have a high false-negative rate — sometimes as high as 50% — meaning a negative result might actually be a positive one, according to the Association of American Medical Colleges.
But even highly sensitive PCR tests can produce a false-negative if you’re tested right after exposure, Harvard Medical School experts say. That’s because your body may not have produced enough virus particles for the test to detect.
“Say you had lunch with a friend who was positive. Day one is the day after you had that lunch. What we found was that if you are tested in the days immediately after exposure, the false negative rate is anywhere from 50% to 100%,” Dr. Lauren Kucirka, an epidemiologist and OB-GYN resident at Johns Hopkins Hospital, told AAMC in October.
Studies have shown that the average amount of time it takes for you to show COVID-19 symptoms after infection is five days — that is if you show symptoms at all.
Peak infectiousness is two days before and one day after symptom onset, a study published in April says.
The chances of getting a false negative test result decrease the more days that pass after possible infection or symptom onset. “Generally speaking, if a test result comes back positive, it is almost certain that the person is infected,” Harvard experts said.
Health professionals with Johns Hopkins said that if you are exposed to a person with COVID-19 and test negative, you should still isolate yourself for at least 14 days. That’s because the disease can still develop between two and 14 days after exposure.