September 28, 2022, 15:20

Do the Omicron Numbers Mean What We Think They Mean?

Do the Omicron Numbers Mean What We Think They Mean?

There’s an urban legend about a Texas man who takes a rifle to the side of his barn and sprays bullets across the wall, more or less at random. Then he finds the densest clusters of holes and paints a bull’s-eye around each one. Later, a passerby, impressed by this display, trots off in search of the marksman. In a reversal of cause and effect, the Texas Sharpshooter is born.

Illustration by João Fazenda

The Sharpshooter Fallacy is often used by scientists to illustrate our tendency to narrativize data after the fact. We may observe an unusual grouping of cancer cases and back into an explanation for it, cherry-picking statistics and ignoring the vagaries of chance. As we muddle through COVID-19’s winter surge, the story holds a deeper lesson about the perils of interpreting data without a full appreciation of the context. Omicron, because of its extraordinary contagiousness and its relative mildness, has transformed the risks and the consequences of infection, but not our reading of the statistics that have been guiding us through the pandemic. Do the numbers still mean what we think they mean?

A coronavirus infection isn’t what it once was. Studies suggest that, compared with Delta, Omicron is a third to half as likely to send someone to the hospital; by some estimates, the chance that an older, vaccinated person will die of COVID is now lower than the risk posed by the seasonal flu. And yet the variant is exacting a punishing toll—medical, social, economic. (Omicron still presents a major threat to people who are unvaccinated.) The United States is recording, on average, more than eight hundred thousand coronavirus cases a day, three times last winter’s peak. Given the growing use of at-home tests, this official count greatly underestimates the true number of infections. We don’t know how many rapid tests are used each day, or what proportion return positive, rendering unreliable traditional metrics, such as a community’s test-positivity rate, which is used to guide policy on everything from school closures to sporting events.

There are many other numbers we’d like to know. How likely is Omicron to deliver not an irritating cold but the worst flu of your life? How does that risk increase with the number and severity of medical conditions a person has? What are the chances of lingering symptoms following a mild illness? How long does immunity last after a booster shot or an infection? Americans aren’t waiting to find out. Last week, rates of social distancing and self-quarantining rose to their highest levels in nearly a year, and dining, shopping, and social gatherings fell to new lows. Half of Americans believe that it will be at least a year before they return to their pre-pandemic lives, if they ever do; three-quarters feel that they’re as likely, or more so, to contract the virus today—a year after vaccines became available—as they were when the pandemic began.

Should we be focussed on case counts at all? Some experts, including Anthony Fauci, argue that hospitalizations are now the more relevant marker of viral damage. More than a hundred and fifty thousand Americans are currently hospitalized with the coronavirus—a higher number than at any other point in the pandemic. But that figure, too, is not quite what it seems. Many hospitalized COVID patients have no respiratory symptoms; they were admitted for other reasons—a heart attack, a broken hip, cancer surgery—and happened to test positive for the virus. There are no nationwide estimates of the proportion of hospitalized patients with “incidental COVID,” but in New York State some forty per cent of hospitalized patients with COVID are thought to have been admitted for other reasons. The Los Angeles County Department of Health Services reported that incidental infections accounted for roughly two-thirds of COVID admissions at its hospitals. (Pediatric COVID hospitalizations have also reached record levels, probably because Omicron’s transmissibility means that many more kids are contracting the virus; there’s little evidence that the variant is causing more severe illness in them, though.)

Clarifying the distinction between a virus that drives illness and one that’s simply along for the ride is more than an academic exercise. If we tally asymptomatic or minimally symptomatic infections as COVID hospitalizations, we risk exaggerating the toll of the virus, with all the attendant social and economic ramifications. If we overstate the degree of incidental COVID, we risk promoting a misguided sense of security. Currently, the U.S. has no data-collection practices or unified framework for separating one type of hospitalization from another. Complicating all this is the fact that it’s sometimes hard to distinguish a person hospitalized “with COVID” from one hospitalized “for COVID.” For some patients, a coronavirus infection can aggravate a seemingly unrelated condition—a COVID fever tips an elderly woman with a urinary-tract infection into delirium; a bout of diarrhea dehydrates a man admitted with sickle-cell disease. In such cases, COVID isn’t an innocent bystander, nor does it start the fire—it adds just enough tinder to push a manageable problem into a crisis.

It is a positive development that we’re able to engage in this discussion at all. With Alpha and Delta, almost all COVID hospitalizations were related to the infection. The situation is different with Omicron—a function both of its diminished ability to replicate in the lungs and of its superior capacity to infect people who’ve been vaccinated or previously contracted the virus. Still, parsing the numbers in a moment of crisis can seem a subordinate aim. Omicron is imposing an undeniable strain on the health-care system. Last week, a quarter of U.S. hospitals reported critical staffing shortages. Many have postponed non-urgent surgeries, and some have asked their employees to continue working even after they’ve been infected. Some states have called in the National Guard; others have enacted “crisis standards of care,” whereby overwhelmed hospitals can restrict or deny treatment to some patients—I.C.U. beds, ventilators, and other lifesaving resources—in order to prioritize those who are more likely to benefit.

But this wave, too, shall pass—possibly soon. At the end of it, the vast majority of Americans could have some degree of immunity, resulting from vaccination, infection, or both. In all probability, we’d then approach the endemic phase of the virus, and be left with a complex set of questions about how to live with it. What level of disease are we willing to accept? What is the purpose of further restrictions? What do we owe one another? A clear-eyed view of the numbers will inform the answers. But it’s up to us to paint the targets. ♦

Sourse: newyorker.com

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