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Faye Flam is a Bloomberg Opinion columnist covering science. She is host of the “Follow the Science” podcast.
For many people, this holiday season feels like a return to 2019, with crowded stores, holiday parties and travel plans. But one COVID concession we should retain at the end of 2022 — and well into 2023 — is the use of rapid antigen tests, especially before attending large gatherings or meeting with people at high risk. Unlike more intrusive measures such as isolation and masking, tests are almost painless. And President Joe Biden’s administration is now again offering four free tests to each household.
But there’s still confusion around these tests — their strengths and weaknesses in different situations, whether they’re just as accurate against the new variants (yes), and what benefit they can have to whole communities.
Yes, many people are tired of buying these tests and aggravated that testing positive can mean canceled plans or kids out of daycare. But one important reason to continue rapid-testing ourselves is to help diagnose people at high risk for severe disease as soon as possible — because the sooner they get the antiviral Paxlovid after developing symptoms, the better. Underuse of Paxlovid has been blamed for many of the ongoing COVID deaths.
Another reason to keep using them is that while rapid tests can miss some cases, testing right before a gathering can vastly decrease the odds of creating a super-spreader event.
That said, it’s critical to know the tests’ limitations. They’re not perfect — and not nearly as sensitive PCR tests. But for many purposes, antigen tests are better because you get results in 10 or 15 minutes. PCR tests are more sensitive but still too slow.
“When your goal is to stop transmission, or to identify when you’re infectious, PCR isn’t the right tool,” said Michael Mina, an epidemiologist at the biotech company eMed, which sells rapid tests.
Mina, who has been a big advocate of home testing since he was an assistant professor at the Harvard School of Public Health, says rapid tests are just as good at picking up the current variants as they were the variants of holiday seasons past. He estimates that rapid tests are better than 90% effective at preventing guests from getting infected and better than 90% effective for preventing your party from becoming a super-spreader event. That’s because the rapid tests are pretty good at detecting people who are carrying what Mina calls “sky-high viral loads” of the kind that could spread to multiple other people. “Does it mean you won’t turn positive tomorrow? No, but it means you’re not highly infectious right then, and that’s the most important thing.”
Some people with very low levels of the virus might never test positive on a home test — but Mina maintains those people are the least likely to spread the virus to others at a gathering. Super-spreading events are more often triggered by people who develop a high viral load but little in the way of symptoms.
Newer at-home tests are now being sold for around $29 each, promising near-PCR accuracy within an hour. That’s a good option for those with the resources, but there’s no reason to let the perfect be the enemy of the good. Lots of suffering can be avoided through judicious use of the cheaper tests.
The biggest weakness of rapid tests is in the instructions, which fail to adequately inform people about when to test, why to test, or how to interpret the results. In a study published earlier this year, researchers found that people did worse using the enclosed instructions than they did with no instructions at all.
To use the test effectively, you have to take the test immediately before your event or gathering — not several days before, as some people still believe. Ideally, you’d take two tests — one a few days before, and another the morning of the event, especially if you were in contact with someone who was sick.
And if you’re coughing or sniffling, it’s always better to stay home, even if you test negative — because you might have flu or RSV, both of which are going around this season and can be dangerous to vulnerable people.
If you test positive, you should isolate. There’s still a lot of confusion here, too. The CDC has issued different guidelines about how long to stay isolated after a positive COVID test — it went from 10 to 5 days without any change in scientific understanding. (The first day of symptoms or a positive test is considered Day 0.) “We find that 50% of people are still really infectious at Day 5,” Mina said. “Around 20 percent of people are still infectious at about 10 days.”
There has been some scientific disagreement about whether it’s really OK to end isolation after five days. Most say it’s OK, if you consistently wear a tight-fitting N95 mask. But there’s little reliable protection from the kinds of loose-fitting cloth and surgical masks most people choose. And to make matters more complicated, exposure time matters — so on Day 8, say, you may be unlikely to infect someone at the supermarket but still able to infect someone you’re sleeping next to for eight hours.
Mina recommends waiting till you get a negative test — though that can be more expensive, if you run out of the free ones. It wouldn’t hurt for the Biden administration to let families have more than four tests.
For many of us, this is going to be an almost-normal holiday season — not because the virus is gone but because we’ve got better tools to deal with it. Nobody wants to be remembered for bringing the unwanted gift of COVID-19 to their friends and relatives.
