Rapid tests can save Christmas, if you can find them

Faye Flam, Tribune News Service

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?’”

And because timing matters so much in COVID-19 detection, rapid tests can be more accurate than PCR for cutting down the risk of super-spreading events at private gatherings. They are also good for anyone planning to visit friends or relatives who are on chemotherapy or immune-suppressing drugs after an organ transplant, or who are over 80, or for whatever reason remain unvaccinated.

On the other hand, if you were infected at a party or restaurant last weekend, took a PCR test on Tuesday, and then go to have Christmas Eve dinner with your grandparents, you’re putting them at risk. A weekend exposure might not show up that early, and by Christmas you could be highly infectious. A rapid test right before your visit could pick up what that early PCR missed.

The PCR test was designed to amplify the viral genetic material — RNA — and can therefore pick up minute amounts. The rapid tests pick up viral proteins, called antigens, and since these are not amplified, it won’t pick up a new infection quite as early, and it won’t keep coming up positive after you’ve stopped being infectious.

Rapid tests are a good first course of action if you feel cold or flu symptoms. If a test is positive, you can start isolating yourself sooner and notify contacts, and soon it will be possible to take advantage of effective antiviral treatments such as Pfizer’s Paxlovid. If you have cold or flu symptoms but test negative, Adalja of Johns Hopkins recommends going to a pharmacy and getting a PCR test before you consider yourself cleared.

If you have no symptoms but a rapid test before an event turns up positive, you’ll have to skip that event no matter what. The experts agree, however, that if you’ve been vaccinated, you can isolate for 5 to 7 days, instead of waiting for 10 days, then take another test. That’s because vaccinated people are infectious for much shorter periods.

Most brands of home tests can detect omicron, though Adalja noted the Food and Drug Administration has reported that three brands — made by Meridian Bioscience, Applied DNA Sciences and Tide Laboratories — don’t work as well at picking up the new variant. Otherwise, he said, all brands are fairly interchangeable.

One problem is the rapid tests are regulated like a medical diagnostic test, and the experts say this has led to an onerous regulatory process that has limited supply and passed the cost to consumers. In much of Europe, rapid tests are regulated as a public health screening tool, which has allowed them to be free and easy to get for months.

The fact that home tests are hard to find and that New Yorkers are waiting in long lines is part of a policy failure. The Biden administration’s 500 million tests won’t be ready to ship by New Year’s, and a website has to be set up and work for people to submit their requests.

How can public health departments track the results of at-home tests? Tests often come with a QR code that makes it easy to report results, but even if people don’t follow through, having the information in the test-takers’ hands is probably more likely to save lives than it would be in the state health departments’ files. The test takers are in the best position to notify contacts, since most state-run contact tracing efforts have been scaled back or discontinued.

Rapid tests might become one part of pandemic response that people won’t mind keeping even after the disease becomes endemic and a big portion of the population stops social distancing and wearing masks. The immune-compromised may be at risk for some time, and testing can allow them to enjoy the kind of social life we all need.