Rapid Covid Tests Are Missing Early Infections, Virologist Says
(Bloomberg) -- Rapid-antigen tests are failing to detect the first few days of Covid-19 infections, a period when people are already capable of transmitting the virus, according to a prominent German virologist.
The shortcoming could undermine efforts to use these fast and cheap tests, already widely deployed across Germany, the U.K. and the U.S., to determine if someone can safely enter a place where people are congregating, Christian Drosten, head of the virology department at Berlin’s Charité hospital, said on a podcast.
It will probably take months before this phenomenon is backed up by clinical data in scientific journals, he said. That’s because, for now, few people who believe they’re healthy are subjecting themselves on the same day to both a rapid antigen test and a molecular test, such as one based on the reverse transcription polymerase chain reaction method, the gold standard for accuracy.
“All I can say right now, as an expert with professional experience, is that this is how it unfortunately looks,” Drosten said. “These dangers exist.”
Typically, an infected person is capable of spreading the coronavirus to others for about eight days, a window that begins a couple of days before the onset of symptoms, Drosten noted. While PCR tests can detect pre-symptomatic infections, they’re not often used with that goal in mind unless someone knows they’ve been exposed to the virus.
Molecular tests are complex and expensive, and people often have to wait a day or two to get their results, which can delay the contact tracing and quarantining that helps break chains of transmission.
Fast and cheap, rapid antigen tests are often hailed as a potential solution to these problems. While they’re not quite as sensitive as PCR tests, there are versions available that are quite reliable, especially if people use them several times a week. Because of their ease of use, they can capture Covid infections that might not otherwise be spotted.
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In a paper published in March -- not yet peer reviewed -- researchers led by Rebecca L. Smith at the University of Illinois at Urbana-Champaign found that, among other things, PCR tests were indeed better at detecting infections early on than a Quidel rapid antigen test. But
the difference narrowed after a few days, along with when the different tests were repeatedly used on people.
Meanwhile, other studies have shown that while PCR tests do detect infections earlier than rapid antigen tests, that advantage lasts for “around a day or even less,” says Jitka Polechová of the University of Vienna, who wrote a review comparing PCR tests to antigen tests.
“Given that PCR tests results are usually not returned within a day, both testing methods are similarly effective in preventing spread if used correctly and frequently,” Polechová said.
According to Drosten, as rapid antigen tests proliferate, it’s becoming clear that people sometimes test negative with these new tools before subsequently coming down with Covid symptoms and testing positive later.
The proliferation of variants of the virus could be one reason for this delay in catching cases, Drosten said.
Another factor could be that the swabs used for rapid antigen tests tend to collect dead human cells teeming with proteins from the virus. However, in the first few days of an infection, most of the human cells are still alive, churning out fresh copies of the virus, making those viral proteins harder to detect, Drosten said.
PCR tests, meanwhile, can register a positive result after detecting a tiny amount of the virus’s nucleic acids.
Even if rapid antigen tests miss the first few days of an infection, they are still powerful tools, Drosten said, because they can show whether someone who is already experiencing symptoms actually has Covid. They’re useful, too, for identifying clusters in settings such as schools, even if it takes a few days to uncover those cases.
The risk comes when governments and businesses put too much trust in the tests’ ability to determine whether someone is capable of infecting others.
“It’s too simplistic to say what’s sometimes being argued politically under the motto, ‘now we can open everything because we have these rapid tests,’” Drosten said.
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