Rapid antigen tests are quick, cheap and easy. But a new study suggests they can also be wrong, particularly about omicron.
Cupertino native Nick LaRocca found that out the hard way, accidentally infecting a friend after trusting test results that were negative.
“I tried to do everything the right way. I thought everything was good, even though I felt really crappy,” said vaccinated LaRocca, 36. “And I got one of my friends sick. That’s the last thing I wanted. … I was literally walking around infecting people not even knowing it.”
In the new study, antigen tests — such as Quidel’s QuickVue and Abbott’s BinaxNOW, which can be easily used at home – missed detection of COVID-19’s omicron variant during the first early days of infection.
The findings, if confirmed, urge against over-reliance on a tool that has become the cornerstone of reopening in-person businesses, schools and social gatherings. While important, testing should be just one part of a broader preventive strategy of masking, good ventilation and small gatherings, said experts.
The unsettling results are based on an analysis of a small number of workers who took daily tests at various settings, including an unnamed San Francisco corporation and New York City’s Broadway theaters.
The study found that 30 people tested negative using nasal-swab rapid tests during the first two to three days of infection. But the virus was detected in those same individuals using the slower, more expensive and more accurate saliva-based PCR test.
In general, people are less contagious during this very early stage. But the study found that in at least four cases, people unwittingly transmitted the virus to others.
“The policy implication is that rapid antigen tests may not be as fit-for-purpose in routine workplace screening to prevent asymptomatic spread of omicron, compared to prior variants,” concluded the study, conducted by the Covid-19 Sports and Society Working Group and Infectious Economics in New York, a consultant to businesses that are trying to keep staff safe. Published on the preprint server bioRxiv, the study has not been peer reviewed.
Although anecdotal reports of inaccurate results from rapid antigen tests abound, the study offers the first real-world proof that the tests lag in the ability to find infection — and supports lab work by the National Institutes of Health that discovered that the omicron variant is better at evading detection than previous variants.
“Early data suggests that antigen tests do detect the omicron variant but may have reduced sensitivity,” according to a Dec. 28 announcement by the U.S. Food and Drug Administration.
The new study doesn’t look at the accuracy of the test when used later in illness – for example, whether you’re still infectious days after recovering from symptoms.
It’s the latest anxiety for Californians trying to make decisions about safety, even as tests remain in short supply.
“If confirmed, that’ll render the strategy of testing before a high-risk encounter — ‘let’s test before we visit grandma’ — less reliable,” according to Dr. Robert Wachter, chair of the department of medicine at UC San Francisco.
When her vaccinated son suffered from flu-like symptoms last week, Ayesha Charagulla, of San Jose, used a home test kit distributed by his school — and got negative results. But a PCR test later confirmed his COVID diagnosis.
“Luckily, we had isolated him from the beginning. But what percentage of students are going to school who are positive for COVID not knowing that they have it?” she said. “I am concerned whether these take-home tests are really reliable.”
During her illness in the days before Christmas, 22-year-old vaccinated Brittany Prock, of Campbell, was puzzled by three different test results. One rapid antigen test showed a very faint line, suggesting infection. She was relieved when another antigen test, taken only one day later, was negative. But a PCR test confirmed infection.
Marya Cunha’s tests also yielded conflicting results. Her results from a test distributed by a Santa Clara school test site were negative, but two different brands told her she was positive. She worried about exposing her mother, who is immunocompromised.
“The school districts are paying for these tests…but it’s confusing,” she said. “And frustrating, not knowing what to trust. I had a card saying I could go places. What’s the right thing to do?”
In Vallejo, Andrew Martinez had the opposite problem: a “false positive” test result. He spent New Years Eve alone in an isolated bedroom, listening as his family tooted celebratory party horns. And he cancelled $400 worth of work appointments. But a later PCR test showed he was negative.
Abbott Labs stands by its BinaxNOW test, saying that its studies prove that the test can detect the omicron variant as well as all other variants and the original SARS-CoV-2 strain. According to a statement by Quidel, its QuickVue test performs as well with omicron as previous variants.
Antigen tests are excellent at detecting people who are most contagious, concluded a new pre-print study from Brigham and Women’s Hospital and Harvard. But accuracy declines in people with low levels of virus.
Immunologist Dr. Michael Mina, a longtime advocate of rapid tests, says the tests work when it matters most — when people are highly infectious.
Scientists are now trying to determine why the tests are missing some cases. Compared to other variants, omicron has been shown to infect faster and more efficiently in our bronchi, the two large tubes that carry air from the windpipe to our lungs, so it may be better detected by a saliva swab than a nasal swab.
In the early days of the pandemic, frequent testing could have dramatically altered the course of the pandemic, said University of Colorado Boulder computer scientist Daniel Larremore, who studies COVID testing. “That virus was a different virus — and controlling it was a different proposition,” he said.
But now, faced with the more transmissible omicron variant, other solutions are needed, said Larremore. “That (testing) strategy is simply less effective now, which means we really do need to focus on ‘layering’ with other interventions.”
The new study “strongly suggests that we will be unable to effectively test our way out of the current surge, even if we each had a week’s supply of rapid tests on the counter,” Larremore said.
“Just like wearing a mask, smaller group sizes or better ventilation, antigen tests are part of a layered approach to mitigating omicron’s spread,” he said. “While they don’t work 100% of the time — and nothing short of complete isolation does — they still help us decrease transmission.”
Yes, you can have COVID-19 but test negative – TwinCities.com-Pioneer Press