In-Depth: People may be infectious with omicron for days before a rapid test turns positive

Rapid at-home COVID test
Posted at 5:58 PM, Jan 05, 2022

SAN DIEGO (KGTV) – The San Diego County Office of Education started distributing hundreds of thousands of rapid COVID-19 tests to local school districts Wednesday, just as a newly released study cast doubt on whether these tests can detect the omicron variant early enough to prevent spread.

The study analyzed data on 30 employees in December. Each employee was tested daily with a rapid at-home test and a laboratory-grade PCR diagnostic.

On average, the study found people harbored infectious levels of omicron for three days before a rapid self-test could detect they were positive.

“What we know so far about COVID is that you’re really infectious before you feel symptoms. And so if there is that lag, then we really need to look closely at our workplace policies to make sure that we’re not setting up, unintentionally, a super spreading event,” said study co-author Dr. Blythe Adamson, an infectious disease epidemiologist and economist.

Dr. Adamson is the founder of Infectious Economics, a firm that helps Broadway shows, sports leagues, and other companies coordinate COVID testing programs.

The study, which has not yet been reviewed by outside scientists, examined the Abbott BinaxNOW COVID-19 Antigen Self-Test and Quidel QuickVue At-Home OTC COVID-19 Test.

The authors said they identified at least four cases where an individual infected someone else during the gap when a rapid test claimed they were negative.

Quidel is based in San Diego and manufactures tests in Carlsbad. A spokesperson said the company was reviewing the new data and declined to comment.

Dr. Adamson said rapid tests are most accurate when used on individuals who are feeling symptoms.

While rapid tests may be less reliable for asymptomatic screening with the omicron variant, “they are still going to be a help because they identify a lot of people that we would otherwise miss,” said UC San Diego infectious disease expert Dr. Robert Schooley, who was not involved in the study.

Dr. Schooley said UC San Diego would be sending rapid antigen tests home to students before they return from winter break as an initial screening method. Those who test negative will be allowed onto campus to undergo a PCR test.

Dr. Schooley said rapid antigen tests remain useful when paired with other public health strategies, but he said the new study underscores that rapid testing alone isn’t enough. He pointed to venues that allow unvaccinated customers who present a recent negative test.

“Is it reasonable to say if you’re not vaccinated, then you can get tested and come to a restaurant or get on an airplane? This is telling you that testing really isn’t a sure-fire way to know that you’re not at risk for transmitting to other people.”

There are several theories about why omicron is harder for these rapid tests to detect than other variants. Antigen tests search for fragments of the spike protein of the virus, which is heavily mutated in omicron. Because of that, it may take more omicron particles to set off the test, Dr. Schooley theorized.

One thing is clear: there is much less time to detect omicron in a person before they are infectious.

With delta, the virus had to copy itself for up to four days before there was enough to infect someone else, Dr. Adamson said. With omicron, it takes just 12 to 24 hours to jump to a new host.