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So you had dinner – indoors – with a friend and the next day you got a call from your dining companion: “I hate to tell you this – but I’ve now tested positive for COVID.” Uh oh, did you catch it from your friend?
Or you wake up in the middle of the night with a scratchy throat, a cough, and a feeling that your head is going to float on your neck. Could it be COVID?
In both of these scenarios, the question of whether or not you have COVID can be answered with a self-test or a PCR test. Many people are opting for the self-test option because now you can easily perform self-tests and get an answer within 15 minutes from the comfort of your home.
But if you take a home test and it’s negative…are you really safe?
This is an issue that the CDC and FDA address in guidelines released yesterday on COVID protocols. And there is a bit of confusion.
The CDC says that if you have been exposed to COVID or are sick with COVID for wear a mask and test 5 days later. If that test is negative, the CDC thinks you’re good to go: “You can end your isolation.”
The FDA, however, now says a negative test is not enough. Here’s what the FDA advised in a “safety communication” published on August 11: If you have symptoms, you should get another test 48 hours later. If you don’t have symptoms, you should take three tests, each 48 hours apart. Only if all of these tests are negative should you consider yourself COVID-free.
And the FDA isn’t alone in thinking that repeat testing is the way to go. Infectious disease experts agree that the only way to be sure you don’t have COVID after a negative home test is to test again – either with another home test or with a more sensitive PCR.
OK… so why am I being asked to repeat my home tests if they are negative?
We get it, repeating the tests isn’t the most practical thing. The problem is that these home tests aren’t particularly sensitive at the onset of a COVID infection.
“There is a recognition that [at-home rapid antigen tests] are less sensitive than PCR tests,” says Dr Apurv Soni, a professor at the University of Massachusetts Medical School. “But it’s not that they don’t work, it’s just that we need to better understand what an effective testing regimen should be.”
The FDA made its recommendations based on a year-long study done in conjunction with the NIH and the University of Massachusetts Medical School that was published preprint on medRxiv August 6.
This study showed that if you take a home test the day you get a COVID infection, there’s a good chance it will come back negative – meaning you could be infectious but a home test won’t. won’t show it yet.
“The data shows very convincingly” that if you take another test or two, you can be pretty sure whether you have COVID or not, says Soni, who was the lead author of this paper.
Soni says: “If you are concerned that you have an infection and you have symptoms, you should take two tests 48 hours apart. If you don’t have symptoms, you should take three tests, one every 48 hours. hours. That’s all.”
So why aren’t home tests good at detecting COVID infections? I thought that was what they were supposed to do.
Alright, so now that we officially know that a single rapid home test isn’t particularly good at picking up an early case of COVID, the question is, why not?
Scientists believe that 95% of the US population has some immunity to COVID due to previous vaccination or infection.
“We are more immune to the virus than in 2020,” Soni says. “So the way your body reacts to the virus is different than it was in 2020. Antigen tests [home tests] are really good at detecting an infection when the viral load is above a certain threshold. But because of the varying degrees of immunity most people now have, “the rate at which the viral load increases in your body is slower.”
However, after a few more days, the amount of virus in your body will likely be high enough to be detected by a home test.
Does the FDA recommendation to repeat the test have anything to do with omicron or the new subvariants?
The good news is that omicron does not appear to have any effect on test sensitivity. “The currently known variants do not affect the result of the rapid test,” explains Meriem Beklizpostdoctoral researcher in emerging viruses at the University of Geneva.
Bekliz is the senior author of an article published August 8 in Microbiological spectrum showing that there is no difference in how well a home test detects the delta variant and the first omicron subvariant, BA.1.
But what about the new BA.4 and BA.5 variants? New search shows that these variants are uniquely adept at evading your immune system, but are they just as good at evading detection in your rapid home tests?
“Rapid home antigen-based tests can detect [all the] sub-variants of omicron,” explains Dr. Preeti Malani, professor of medicine at the University of Michigan. “Home tests don’t rely on the spike protein, where the mutations occur in the variants.”
The spike protein is what our immune system looks for to identify and neutralize COVID. This is what many vaccines are based on. And that’s why changes in the spike protein have allowed new variants of COVID to somewhat evade detection by our vaccine-primed immune systems.
Home rapid antigen tests detect a different protein, the nucleocapsid. And all omicron sub-variants have the same version of the nucleocapsid. What does that mean? “Theoretically, there should be no difference in detection sensitivity between omicron BA.1 and its sub-variants [including BA.4/BA.5]“, says Bekliz.
What if after two tests 48 hours apart, I’m still sick and still negative?
So yeah, maybe you just don’t have COVID. “There may be an alternative virus or even a bacterial infection like strep throat, causing disease and not COVID,” Malani says.
However, two negative tests 48 hours apart are not 100% definitive. “Apart from repeating the test at home, you can also consider testing with PCR,” says Malani.
PCR tests are more sensitive to lower viral loads than rapid home antigen tests. So if you take a PCR test while you’re sick and it comes back negative, you can be pretty comfortable knowing it’s not COVID.
However, just because you don’t have COVID doesn’t mean you can come to the office. “If you are sick, whether from COVID or another virus, you should stay away from others,” Malani says.
Does that really mean that if I’m having dinner with a friend who then tells me he has COVID that I have to test three times in six days?
This is where things get a little complicated. Even Soni admits that testing for so long “is a bit impractical”. But he also says, “If you re-test [ahead of whatever social event you want to go to] it gives you the utmost confidence that you are not infected and therefore do not pass the infection on to others.”