As terrifying as the rapidly spreading omicron variant is, fewer Americans should have to spend the holidays alone this year out of fear of contracting Covid-19. Not only do we have life-saving vaccines. For many people, rapid tests can effectively flag those who are likely to be infectious, allowing others to gather safely.
Sadly, many people won’t be able to get those tests when they’d do the most good—right before a holiday visit. We can hope that President Joe Biden's pledge to get 500 million free tests mailed out to any US household that requests one will help remedy the underuse of an important pandemic control tactic.
There are multiple benefits to the sorts of home Covid-19 kits that the administration has promised to deliver. They can prevent outbreaks and save lives. They can also allow people in fragile health and at high risk of contracting severe Covid-19 to enjoy necessary human contact. It’s tragic that the cost and scarcity of these tests have prevented such interaction, given they’ve been around for months.
A number of experts say it’s misleading to compare the sensitivity of these quick tests—technically called antigen tests—to PCR tests, for polymerase chain reaction, which are considered the gold standard. “You can't really quantify the accuracy because it depends upon the question that you're asking,” said Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security.
PCR tests can identify almost everyone who has any viral genetic material in their system, even if that person is asymptomatic or no longer infectious. Studies have suggested that rapid tests catch more than 90% of cases in people with symptoms; they aren't nearly as sensitive at picking up asymptomatic infections—with the more damning studies claiming they flag fewer than 50% of cases picked up by PCR.
But that misses a crucial point: Rapid tests still pick up most infections when they are in the contagious stage. Most of the missed infections, in other words, are in stages too early or too late for the virus to spread to others.
“Home testing is a holy grail we’ve been aiming for since the beginning of the pandemic,” said Nathaniel Hafer, a microbiologist at the University of Massachusetts Medical School. He reiterated that it’s hard to quantify their accuracy because it depends on what you’re trying to learn and when you take the test.
One booster of rapid testing is Michael Mina, a pathologist who left the Harvard School of Public Health in November to join eMed, a testing company. While individual PCR tests are considered 98% sensitive—meaning less than a 2% false negative rate—as a screening tool to monitor Covid-19 spread, Mina and two co-authors argued in an article last year in the New England Journal of Medicine that they are failing, catching fewer than 10% of cases.
Timing matters a lot because infections with SARS-CoV-2 are so dynamic. The virus tends to incubate at levels undetectable by any test for a couple of days, before growing explosively, often moving from undetectable levels to infectiousness within a span of 12 hours. Omicron may move even faster.
Rapid home antigen tests let you get results within minutes of an event or meeting. “The antigen tests are very good at detecting virus in the amounts that are necessary to infect somebody else,” Adalja said. “You're asking, ‘Am I a danger to others?’”
Bloomberg
