Rapid antigen tests have limited use, don’t prevent transmission of COVID-19 and are not a replacement for the vaccine, according to British Columbia’s top doctor.
“It’s not a tool that can be used to prevent transmission. It’s a tool that helps you detect transmission, perhaps, earlier than you would have in that type of a screening setting,” Dr. Bonnie Henry said in response to a question about rapid testing programs for unvaccinated individuals during a virtual town hall, which was hosted by the University of the Fraser Valley (UFV) on Dec. 8 and also included remarks from Dr. Réka Gustafson, the province’s deputy provincial health officer.
Rapid antigen tests (RATs) are not widely available to the public in British Columbia — unlike in Nova Scotia or New Brunswick — even though the federal government has provided the province with more than 3.2 million tests.
As a result, B.C. health officials have faced increasing pressure in recent days from various doctors and groups urging them to provide RATs at no charge to the public, particularly as infections rise on Vancouver Island.
During the virtual town hall, Henry explained that RATs don’t prevent transmission because when someone tests positive, it is “in a sense too late” as they’ve likely been somewhere in the community, potentially exposing others to the virus. She also said any notion that RATs replace vaccinations is “false” and that such tests aren’t nearly as effective when the individual is asymptomatic and vaccinated.
“There is very limited utility of the rapid antigen tests or even the lateral flow tests in asymptomatic people, particularly vaccinated asymptomatic people,” said Henry. “The yield is very, very low for asymptomatic people vaccinated or unvaccinated.”
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British Columbia has only used about nine per cent of the 3,200,306 rapid antigen tests provided to them by the federal government, according to the latest data on Health Canada’s website. The province does make RATs available to those in long-term care and assisted living facilities, and businesses can access them through the Safe Screen BC Program.
Henry explained that the province uses RATs and also provides “rapid access” to polymerase chain reaction (PCR) testing to investigate outbreaks and contacts of individuals who may have been exposed to the virus mostly in communities where “access to regular PCR testing” takes a lot of time.
“It has a purpose in some places. Where we have been using it and where we will continue to use it is to be able to rapidly assess people with symptoms or who have been exposed. So, where your pretest probability is higher, that somebody may be carrying the virus,” she said.
However, the province’s top doctor did not specifically explain why rapid antigen tests are not available to the wider public. Instead, she said there is a global shortage of “some of these” rapid tests and that some of the tests approved by Health Canada are not packaged well enough to take home.
“Health Canada has probably recently approved four different at-home test kits but only one of them, so far, has been available and it comes in packets of 35 tests with one sort of little dropper thing,” she said. “They’re not really packaged right now to be able to actually give them to people to take home.”
Henry did allude that the province is looking at making at-home testing kits available in the future.
“Where I’m hopeful we might get some benefit in the future is the at-home tests that are now becoming available, the lateral flow tests, they’re used quite extensively in the U.K., but where they can be used to help people make a decision about what to do,” she said. “That’s if I am symptomatic, or my child has symptoms, can this help me decide whether I need to stay home and get another test, a more accurate PCR test if it’s positive? So it’s evolving.”
What exactly is a rapid antigen test anyway?
Rapid antigen tests, according to the U.S. Center for Disease Control, are a specific type of test designed to detect the presence of SARS-CoV-2 in an individual. They can be administered as self-tests and can produce results in as little as 15 minutes. However, RATs are not as accurate as polymerase chain reaction (PCR) tests, which is considered the “gold standard” when it comes to testing.
Dr. Lyne Filiatrault, who was formally involved in British Columbia’s SARS response in 2003, says rapid antigen tests help people figure out whether they are safe to be around others.
“The rapid antigen test is a quick, easy answer to the specific question about how safe are you to get together with others. But it’s not, by all means, a definitive diagnostic test for COVID,” says Filiatrault.
“They look very much like a pregnancy test, except you don’t do the pee test, you put the swab up your nose.”
Health Canada has authorized 24 rapid tests and encourages their use — to a certain degree — because they “provide an extra layer of defence” against the spread of COVID-19. They also recommend that anyone who receives a positive test after using a rapid antigen test book an appointment for a PCR test.
“Those receiving a positive rapid test result should seek a PCR test as soon as possible for a confirmatory diagnosis. This also allows local public health authorities to identify the nature of the virus and track variants of concern. Please follow all isolation and quarantine advice given by public health officials while you wait for test results,” reads a statement on Health Canada’s website.
Filiatrault says one of the benefits of a rapid antigen test is that it does allow someone to isolate right away if they test positive, unlike a PCR test, where the results can take days, depending on the laboratory.
“If it is positive, you can isolate right away, there’s no waiting two, three days when the lab is overwhelmed … you can already isolate, make arrangements to get to a PCR testing centre,” says Filiatrault, who is also a member of Protect our Province B.C.
Another great thing about rapid antigen tests, according to Filiatrault, is that they can detect asymptomatic patients regardless of vaccination status — as long as the individual is infectious enough for the test to work properly.
“Rapid antigen tests will pick up [whether] you are infectious with COVID no matter whether you’re vaccinated, no matter whether you have symptoms,” she says. “The only limiting factor is you need to reach a viral threshold in order to have a positive rapid antigen test.”
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Both Health Canada and the U.S. Center for Disease Control discourage against regular use of rapid antigen testing on asymptomatic vaccinated individuals, suggesting that doing so can produce false-positive results.
“Given the effectiveness of vaccines, all other things being equal, the pre-test probability of COVID-19 infection in people who are vaccinated will be lower than in people who are unvaccinated. This lower pre-test probability reduces the positive predictive value of testing, increasing the chance of false positives. False positives can be confirmed by repeating the test or using a more accurate test,” reads a statement on Health Canada’s website.
However, Filiatrault stressed that the rapid antigen test is one part of a broad set of tools that can be used to prevent and reduce transmission. She also says vaccinated individuals still can transmit COVID-19, the difference being that they typically don’t transmit the virus for as long as someone who is unvaccinated.
“When you’re vaccinated, you don’t transmit for as long a duration as an unvaccinated people person,” Filiatrault says. “So, with Delta, we found that the vaccinated individuals still develop a high viral load, and were able to infect others. But they did this for a reduced timeframe, unlike the unvaccinated.”
Filiatrault also says with COVID-19 being an airborne virus and the arrival of the Omicron variant, rapid antigen tests should be made more widely available because they can help reduce transmission by providing individuals with the information they need to make informed decisions.
“We’re going to need right now to add every single layer because there is not one way that is 100 per cent,” she says.
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