What are rapid antigen COVID tests and why aren't we using them more?
How do rapid antigen COVID-19 tests work? Why did the US government just pledge to buy 280 million of them? And is that even enough to help curb the spread of the virus?
As part of President Biden’s recent "Path out of the Pandemic" plan, increased testing was flagged as an important part of containing the spread of COVID-19. The plan indicates nearly US$2 billion will be spent procuring 280 million rapid COVID-19 tests. Plus, major retailers will be required to sell these tests at cost, approximately 35 percent less than current prices.
These numbers certainly sound big – 280 million tests is a lot. But what is the best way to use these rapid tests? How accurate are they? And is the government underestimating how many tests will actually be needed to make a difference?
What is a rapid antigen test?
The gold-standard test for SARS-CoV-2 is known as a reverse transcriptase-polymerase chain reaction (PCR) test. This test, involving the infamously fun experience of having a swab inserted deep into one’s nasal cavity, looks for viral RNA.
PCR tests are incredibly accurate. So much so they can pick up minute traces of viral RNA that may only result in innocuous asymptomatic infections, or even offer positive diagnoses weeks after a person has passed their infectious phase.
But PCR testing is also time and energy intensive, requiring samples to be sent to diagnostic laboratories. At best, the results of a PCR test can take several hours. And at worst, when labs are overwhelmed, it can be days before a result comes back.
Rapid COVID-19 tests, on the other hand, sacrifice a degree of accuracy for speed and simplicity. These tests look for proteins on the surface of viruses called antigens, or antibody generators.
A rapid test contains a panel of antibodies designed to respond to SARS-CoV-2 antigens, and they can return a positive or negative result within around 15 minutes.
How accurate are they?
Rapid antigen tests are without a doubt not as accurate as PCR tests. However, rapid antigen tests are incredibly accurate at catching cases at their most infectious point.
Rapid tests require substantial volumes of virus to be present in any given swab. This means these tests can’t confirm someone is not currently infected with SAR-CoV-2 but instead can confirm whether a person may be infectious at the time of taking the test.
“Rapid tests are designed to identify cases with a high enough viral load in the nasal passage to be transmissible – not to diagnose all COVID-19 cases,” explains public health expert Zoe McLaren, from the University of Maryland. “The Abbott BinaxNOW rapid antigen test may only detect 85 percent of the positive cases detected by PCR tests. But the key is that published studies found that they detect over 93 percent of cases that pose a transmission risk, which is what matters most for getting the pandemic under control. Ellume correctly identifies 95 percent of all positive cases, and Quidel QuickVue accurately identifies 85 percent. All three tests correctly identify upwards of 97 percent of all negative cases, regardless of symptoms.”
So what is more important in a diagnostic test for COVID-19, speed or sensitivity?
One article published in early 2021 suggested widespread and frequent use of rapid antigen tests could more effectively help contain the pandemic compared to solely relying on slower PCR testing.
The study modeled viral spread across a population taking into consideration several variables, including test sensitivities, frequency, and sample-to-answer reporting times. The research found test sensitivity is less important than frequency of testing and the time it takes for a test to deliver results.
“These results demonstrate that effective screening depends largely on frequency of testing and speed of reporting and is only marginally improved by high test sensitivity,” the researchers wrote. “We therefore conclude that screening should prioritize accessibility, frequency, and sample-to-answer time; analytical limits of detection should be secondary.”
So, essentially, a less-sensitive but simple-to-use test administered frequently would be more helpful in managing a pandemic than a slow and difficult-to-access but highly sensitive test.
How to best use rapid antigen tests
Both the FDA and the CDC recommend serial testing when using rapid antigen tests. Because these tests require a relatively high viral load to return a positive result it is possible to test negative for SARS-CoV-2 but still be infectious. To overcome these false negative results it is recommended people test themselves at least twice over the course of a few days to be sure of a negative status.
Earlier this year a massive publicly funded program in England offered every citizen free rapid testing kits. The goal of the government funded program was for every citizen to test themselves at home twice a week.
It is estimated nearly 100 million tests have been distributed since the program began, and around 620,000 positive COVID-19 cases have been linked to the diagnostic program. The UK government recently announced it would be stopping the free rapid test program from the start of next year.
In the US, President Biden’s recently announced plan to ramp up access to rapid testing has been criticized by some as too little, too late. An editorial by Eric Topol and Daniel Oran from the Scripps Research Institute called for the US government to drastically increase rapid testing manufacturing, arguing an order for 280 million tests at $7 a test is far too few tests for far too much money.
“Instead, the U.S. should plan for a several-month surge of rapid testing, aiming to test a substantial proportion of the population at least twice a week,” Oran and Topol write in the editorial published by StatNews. “That would require around one billion tests a month. And the goal should be to get the price per test much closer to one dollar.”
Polling earlier this year found nearly 80 percent of Americans would regularly test themselves for COVID-19 at home if rapid antigen tests cost around one dollar. However, only 33 percent of those polled were likely to engage in frequent home testing if the tests cost upwards of $25.
Currently, even with the White House’s “sell at cost” decree for major retailers, the cheapest price for a two-pack rapid antigen home test is about $15. In Germany, rapid tests can be bought from local grocery stores for less than one dollar. In Singapore vending machines are being set up across the city allowing those flagged as close contacts of positive cases to pick up free rapid tests.
Harvard epidemiologist Michael Mina has long advocated for widespread and frequent rapid antigen testing. He says the cost of these tests is high in America due to onerous regulatory burdens that have limited production.
“The reason it's so expensive here is because we have no market competition,” Mina said in an interview with NPR. “The actual cost of the devices is very inexpensive. But what we need is market competition. We need more producers to get into the United States market. And to do that, we have to release some of the regulatory barriers that we have thus far had.”