Reality check: COVID-19 rapid antigen tests and new variants
New COVID variants throw a wrench into the mix, but don't toss your tests yet!
You likely know at least one person for whom rapid antigen tests didn’t deliver. Maybe they were negative for three days, then, when they were finally feeling better… boom! The dreaded double line. Maybe it took more than three days.
It’s natural to feel discouraged by stories like this, and to ask yourself “why bother?”. I’m here to remind you that there are also many success stories out there, including my own.
Rapid antigen tests have repeatedly helped us flag COVID-19 infections in time to isolate and spare other family members - not to mention those outside our family. COVID-19 has entered our home four times in the last 13 months, and in three of those four cases, nobody else got it, thanks to rapid tests and the subsequent precautions we took. Often, the initial symptoms were so subtle that we were very surprised to see a positive COVID-19 test.
So, my message to you today is this: don’t throw out the baby with the bathwater. Rapid antigen tests have major limitations, but they can still be a valuable tool - if we understand their limitations and use them wisely.
Got COVID? See this post on my Instagram page for tips on containing COVID-19 at home. If you are willing to accept a small amount of risk, you don’t have to fully isolate with zero contact for 10 days, as was common practice back in the early days.
How well do rapid antigen tests work with current variants?
Unfortunately, there is no simple answer to this question, because real-world performance is complicated. It depends on the variant, the test brand, the user, their technique, and more! It's messy.
Before variants came along, rapid antigen tests worked very well, especially for symptomatic cases, or asymptomatic cases that were bursting with virus.
Show me the data! A nationwide US study during the Delta and early Omicron waves reported:
Symptomatic cases: A single rapid antigen test detected 60% of symptomatic cases on Day 0, and 90-95% of symptomatic cases on Days 2-4 (Day 0 = first positive PCR). Overall, rapid antigen tests detected the virus 83% of the time with one test, and 93% of the time with two tests 48 hours apart (in symptomatic cases, Days 0-6).
Asymptomatic cases: A single rapid antigen test detected only 9% of asymptomatic cases on Day 0 yet caught 75% of asymptomatic cases on Day 4 (Day 0 = first positive PCR). Overall, rapid antigen tests detected the virus 34% of the time with one test, 56% of the time with two tests, and 69% of the time with three tests 48 hours apart (in asymptomatic cases, Days 0-6).
Nowadays, the performance of rapid antigen tests is less predictable. Many people still get their first positive right away, but it’s not uncommon for 3 days or more to pass before the first positive test - often, when symptoms are receding!
We don't fully understand all the reasons for this shift, but there are at least three factors at play. The first is changing fit, between virus and test. You can think of antigen tests as Cinderella's slipper and the viral protein target as Cinderella’s foot. If Cinderella’s foot changes (let’s say she gets pregnant), the fit of every "shoe" (test brand) will be affected differently. The other key factors are changing viral biology (where and when it grows in your body), and changing host biology (immunity).
Citizen science insights
Here’s a snapshot of current real-world performance of rapid antigen tests thanks so many of you. Since it’s too early for publications, I turned to citizen science and polled followers on my Instagram page about their experiences in the last 2 months.
The good news is that for half of us, antigen tests are rising to the occasion and giving us a positive result within 0-1 days of symptom onset. The bad news is that for a nearly a third of people, it takes 3 days or longer to get the first positive antigen tests. It’s unclear what drives this variation from person to person. Likely, the answer involves test brand, swabbing technique, symptom status, and level of immunity (which I’d love to survey in the future!).
Nerd note: Two of the hallmarks of good science are reproducibility and open acknowledgement of limitations. Results from this citizen science poll should only be seen as a rough guide, due to self-reported data and the many variables at play, mentioned in my footnote. As for reproducibility, I ran the same poll on the Instagram page for Those Nerdy Girls (also on Substack!) and the results were very similar.
Should I still use antigen tests before a gathering event?
My answer: don't let perfect be the enemy of good! Using rapid antigen tests before a gathering will catch *some* infectious cases, but they won't catch them all. You can think of them in the same way as masks - a layer of protection, that won’t make you “bulletproof”.
I hesitate to put a number on how many cases they catch, because it depends on so many factors. What we do know is the hotter the case (higher viral levels), the higher the odds that a rapid test will light up, regardless of symptoms (see review study). Given that antigen tests are not a perfect filter to keep out infectious cases, it’s important to recognize this risk, especially when gathering with people who can’t afford to be sick.
One of the best ways to keep your gatherings safe is simple, yet frustrating: err on the side of staying home even if you’re feeling just a little off, if the stakes are high.
Tips for testing wisely
Here are some tips for testing wisely given the limitations of the current situation:
👉 Optimize timing: If you only have one test available, consider waiting 2 days from symptom onset to give the virus ample time to replicate. Be sure to and play it safe in the meantime.
👉 Repeat if negative: If you suspect you may have COVID but test negative, re-test again in 48 hours apart. A single negative test is *not* conclusive.
👉 Consider a dual swab: A combined swab (first throat, then nose) may boost your chances of detecting COVID-19 early. Why? Because the SARS-CoV-2 virus can sometimes replicate first in the throat before it takes off in the nose.
Note: Combined swabs are not authorized by the FDA, primarily due to safety concerns, but have been successfully used in the UK, Israel, and parts of Canada. For more on the performance and use of combined swabs, see this great review by CADTH, the Canadian Agency for Drugs and Technologies in Health.
Thanks for joining me in choosing science as your guide!
Best,
Chana
Chana Davis, PhD @fueledbyscience
Other resources
Study on how COVID-19 variants impact antigen test performance
FDA information on variants and rapid antigen tests (FDA)
FDA communication on reducing false negatives with repeat testing (FDA)
My article about rapid testing and variants(Those Nerdy Girls)
Expert opinions on rapid testing and variants (Huff post)