This post is about the over the counter Antigen tests that a lot of us have used in the last few months, and whether they really work.

As we’ve discussed, BA.5 is re-infecting lots of people, even those who’ve had Omicron just a few months ago. Those with recent cases generally have a mild case the second time around, even with BA.5

I just got back from summer camp with a bunch of high school students. I knew I could get COVID again, but since I had Omicron in January, I figured I’d be OK, even if I got BA.5. One of our counselors got sick while we were there, and tested negative for COVID, with an antigen test. I got sick on Sunday morning, after we were back, and also tested negative for COVID using an antigen test. On Sunday I just had a sore throat, Monday and Tuesday morning I had fatigue, sore throat, and just a little coughing. My sore throat felt very similar to the one I had in January, kind of like a weird heart burn, so I tested myself again on Monday. Still negative.

Meanwhile, the other counselor tested negative again when he got home, but then tested positive after that. His wife too. So he had 2 negative tests before testing positive.

So did I have COVID again? I still don’t know. I’m at the “just getting over a cold” stage right now and am feeling pretty normal. I had 2 negative tests and I’m not going to test myself again.

Doing a good scientific study is a slow process, and we’re probably unlikely to get good scientific articles on how well the antigen tests work for every new variant. The only articles I could find on BA.5 and antigen tests were from Slate and CNET. I usually don’t bother to read popular level articles on COVID since journalists often get things wrong, but that’s all we have. Both basically say that yes, they work, but you may need to take several tests. Well, that’s kind of like saying they don’t work very well.

How Antigen tests work: There are 2 kinds of tests that use antibodies for testing. One is an Antibody test. This kind of test looks for human antibodies against SARS-2 proteins. It actually determines whether you’ve been infected at some point in the past. IgM antibodies appear after a few days and persist for about a week. IgG antibodies appear after about a week and persist for weeks or months. So a positive SARS IgM tests says you basically have COVID right now, while a positive IgG test you have it now, or had it weeks or months ago. An IgG test is not useful if you want to know if you have COVID now.

An Antigen test detects actual SARS-2 proteins. They usually detect the Nucleocapsid protein (N), a protein on the inside of the virus which does not mutate rapidly. In theory, these tests should work well even if the Spike protein changes, which is by far the most common changes in new variants. They are not usually thoroughly tested with each new variant.

The Slate article suggests the tests may not be working as well because the BA.5 variant may not be as present in the nostrils as previous variants. Scientists know that a Nasopharyngeal (back of the nose/throat) site is better, but patients far prefer a nostril test. So this may be why the tests are not working as well for BA.5.

Antigen tests are less sensitive than a PCR test, but they are far cheaper, faster, and easier to use, which is why they are so common now. If you really need to know if you have COVID, a PCR test is the way to go. I frankly am pretty disappointed that a clearly symptomatic person can test negative for COVID right now with an Antigen test.

All this to say, if you have cold-like symptoms in the next few weeks, you’re better off staying at home and laying low for a few days, even if you have a negative Antigen test. You may actually have COVID.

Happily, if I indeed have COVID now, it’s a very mild case.

Don’t fear, but be smart,
Erik