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No, BA.2 is not prone to "false negatives" on rapid tests

Some Twitter users claim that omicron variants BA.1 and BA.2 are prone to "false negatives" on rapid tests. Experts say that's not true.
Credit: Patrick Daxenbichler - stock.adobe

WASHINGTON — The omicron variant of COVID-19 became the dominant strain in the U.S. in late December 2021. Now, its subvariant, BA.2, has taken the reigns.

From March 20-26, BA.2 accounted for nearly 55% of COVID-19 cases in the U.S., according to estimates from the Centers for Disease Control and Prevention (CDC). That’s compared to about 40% of cases attributed to BA.1, the original version of omicron.

BA.2 has been lovingly nicknamed "stealth omicron" because it can be difficult to differentiate the variant from delta on PCR tests. But those details can sometimes get lost in headlines and social media posts, sewing confusion around what that nickname really means.

Some tweets claim outright that the omicron variants are prone to false negatives on at-home rapid tests. 

THE QUESTION

Are omicron BA.1 and BA.2 prone to false negatives on at-home rapid antigen tests?

OUR SOURCES

THE ANSWER

No, there is no evidence that the omicron variants are specifically prone to false negatives. Our experts say the likely explanation is that people may be testing too soon before the virus has replicated enough to show up on an antigen test. 

This does not mean the tests don’t work, but our experts say it likely means that in vaccinated and boosted people, the immune system is preventing the virus from replicating as quickly. They recommend testing 3-5 days after known exposure, then if that is negative, test again a few days after that. 

WHAT WE FOUND

While the COVID-19 virus continues to mutate, it has not evolved to be able to evade a rapid antigen test. In fact, Dr. Gronvall explained the rapid antigen tests were specifically designed to screen for a part of the virus that is unlikely to change.

“There are a lot of differences between omicron and delta and alpha, but not in the part that the tests look for, and that's by design,” she said. 

This is supported by a December 2021 study that found that antigen tests detect omicron and delta infections at similar rates and a March 2022 preprint which concludes that rapid antigen tests do not perform worse in people infected with omicron versus delta.

Dr. Binnicker pointed out that not all at-home rapid antigen tests work the same way or at the same quality. In fact, a February 2022 study found a wide range of abilities to pick up delta and omicron infections when comparing nine tests on the European market. 

He added that what could be driving these claims is that people may be testing themselves before there’s enough virus in their nose to turn up positive on a test. 

“I don't think they've outsmarted the tests, it really comes down to how much a virus is present in a person's nasal passage,” he explained. “If it hasn't been long enough for the virus to kind of incubate and reach higher levels, those tests are going to be negative more often.”

This isn’t necessarily a characteristic of omicron BA.1 or BA.2; this is how the rapid antigen tests work with all strains of COVID-19. 

RELATED: Which COVID-19 test should you take?

But if that’s true, why does this appear to be happening to so many people now, when the omicron variants are the most prominent? It seems like a lot of people, this writer included, showed symptoms for several days before finally testing positive. Dr. Gronvall said that’s likely a result of being vaccinated against COVID-19. 

The vaccine gives your body the instructions for spotting and fighting SARS-CoV-2. If you get infected, your immune system snaps into action to try to kill the virus before it can replicate. The cold-like COVID symptoms that might show up are a result of that fight, not of the virus itself. So those couple days of coughing and testing negative is not a product of something special about omicron, but something special about being vaccinated.

“Your body goes into high gear, you will start to feel sick, and a lot of this is your body making it a terrible place for a virus to grow,” Dr. Gronvall explained. “If you had not been vaccinated, it would take your T cells and B cells five or six days to recognize that there was a problem and start responding to the virus.”

So here’s the information roundup: We know that rapid antigen tests require a certain level of viral load in the nasal cavity to register as positive. We know that might take a few days because the virus replicates slower in vaccinated people. But what we don’t know for sure yet is the threshold of viral load at which someone becomes contagious.

That means those last days when you might be coughing but are still negative on an at-home test, it’s not entirely clear whether you’re contagious yet. Dr. Gronvall explained that’s because the studies needed to get an answer on contagiousness are not ethical with a virus this new. 

“Eventually, if you can find a person who you know they're not going to get long COVID and you know that you can treat them quickly, then you can start to do these kinds of studies,” She said. “We can't do that with COVID yet, it's unethical, because people might have debilitating symptoms for months or years to come. So right now, we just have these gaps that can't really be easily filled.” 

While a quantifiable threshold for contagiousness is up in the air, many experts still believe that antigen tests are the best tool for determining in-the-moment contagiousness. Johns Hopkins Senior Scholar Dr. Amesh Adalja told WUSA9 via email that someone is unlikely to be contagious if they test negative on an antigen test. 

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