The Best News About Omicron May Not Be About Severity

Photo: David McNew/AFP via Getty Images

The new news about the severity of Omicron is good news. But it may not be the most important news. That is about the size and speed of the wave, which seems potentially now shorter and faster to pass than feared just a week or two ago.

As with all data about the new variant, massive caveats apply — among the astonishing things about this wave of the pandemic is the speed at which we are all studying it, even faster and more transparently than transpired in the winter of 2020, such that now epidemiologists and the public are learning about Omicron’s distinctive features in real time as the very first clinical cases travel their course. But the news about Omicron’s relative pathogenicity is now coming from many countries outside South Africa, and though it is partial and preliminary, it also appears quite unanimous: Whether from inherent reductions in severity, population-level immunity, or some other factor, the new variant is causing less severe illness wherever it is popping up. The estimates fall along a range — hospitalizations reduced by 80 percent in South Africa, for instance, by 60 percent in Scotland, but only 40 percent in England. That the range is so large indicates both a lot of remaining uncertainty and the possibility that the wave may look quite different from country to country given local immune protection and other factors. But altogether they suggest that — at least in those countries with Omicron well underway — we should expect the waves to be significantly less brutal, per case, than earlier ones. But before we celebrate: How much does a reduction of, say, 50 percent matter?

The question may seem ridiculous on its face — of course, severity matters, and if the most generous estimates bear out, it could mean as much as 80 percent less severe illness given the same number of cases. But that “given” of similar caseloads is not at all a given — at least a week or so ago, it was the working assumption of most studying the early stages of the new wave that it would produce far more cases than we had seen at any earlier stage of the pandemic. In fact, almost all early projections of Omicron’s spread, even the conservative ones, suggest such massive waves that a reduction in severity of that size — a halving — would still mean a much bigger toll than in previous waves. That is because, as John Burn Murdoch put it, “a small percentage of a large number can still be a large number.” It is why so many epidemiologists have been warning that even in a “best-case” outcome for Omicron’s severity, the ultimate toll could be very large indeed. Which means that, though it has been suggested again and again in the early stages of Omicron that the big unresolved question is about the relative severity of the new variant, the size of the wave is probably the more important variable.

This may sound abstract, but using even hypothetical numbers drives the point home very clearly. Take a strain that is half as severe but reproduces twice as fast — the effect doesn’t even cancel itself out because, over the course of weeks or months, the wave doesn’t just double but grows many times the size (this is what epidemiologists and VCs both mean when they say the public doesn’t understand exponential growth). When I spoke to Trevor Bedford of Seattle’s Fred Hutchinson Center for Research last week, he told me a crude model of Omicron spread suggested that it could reach 90 percent of the population. The trajectory in South Africa, he told me, indicated the variant wouldn’t be quite that prolific, but “I can easily expect a 50 percent attack rate,” he said. A 50 percent attack rate implies 160 million American cases. Cut severity in half from our experience with Delta, and if we truly get 160 million cases, it could mean almost a million deaths just from Omicron. If Omicron continues to spread preferentially in the vaccinated and already-protected, that figure could be much lower — but still harrowingly high. And Bedford recently told Helen Branswell that it wasn’t just the U.S. that could see a 50 percent attack rate but the whole world — 4 billion cases, which implies an enormous, even unprecedented, amount of global illness and death, even given reduced severity.

That is why the size of the wave is probably more important than the severity of the strain. And why the most important thing to sort out right now about the ongoing wave is why South Africa turned so quickly and so sharply. And why it may be very good news that in many places the Omicron curves appear to be already turning or perhaps about to: in Norway, for instance, total case numbers appear in decline, while elsewhere in Europe and the U.K. case growth is slowing or even plateauing. Summarizing the recent data on Twitter, Bedford wrote, “Omicron wave will be fast and intense across geographies, but is expected to burn through quickly,” though he also warned that, “even if individual potential of severe outcomes are lower for Omicron, regions with already strained hospital capacity are particularly at risk.”

Of course, none of these waves are yet over — even in South Africa, the country appears barely past its peak with a long way to go until zero. Some of the data may be noisy, and the peaks may prove more elusive — or even relatively far. Even those that appear to be cresting may stall into long plateaus, as the U.S. and U.K. did with Delta, or even peak again — there is still so much we don’t understand about the dynamics of pandemic spread it is hard to say anything for sure. And the bundle of countries whose data is taking an encouraging turn is not necessarily globally representative, since they are either well-vaccinated or, in the case of South Africa, well-protected by previous exposure — though if Omicron does prove to be primarily a “reinfection variant,” that may make these countries outliers in the global experience, not because they are unusually protected, but because they are the natural sites of breakthrough spread.

There are additional caveats and complications, too. In terms of daily cases, many local areas (like New York City) and even some countries (like France and the U.K.) are already reporting higher numbers than they have seen at any previous point in the pandemic — these are, indeed, massive caseloads. And we are still observing those cases progress — through the natural clinical course, for individual cases, and through the population as a whole, for the wave as a whole — which means that our picture of the ultimate impact of the wave is still evolving, too. The hospitalization data in New York, for instance, is encouraging, for instance, while the data from the U.K., a bit farther along in their wave, gives a more mixed picture, and while in total it appears deaths in South Africa won’t even approach the height of the Delta wave, even there it appears that among those already hospitalized with Omicron the severity of the disease may be no lower than in previous waves. This is especially concerning because South Africa’s dramatically reduced initial hospitalization rate with Omicron, a cause for relief and celebration the world over, marked a reduction from a terrifyingly high rate of over 19 percent in the last wave; for comparison, the U.K. hospitalization rate with Delta was barely 2 percent, about where South Africa’s Omicron rate appears to be, which suggests that some of the gains observed in South Africa from additional immunity and casually attributed to reduced Omicron severity may have already been gained in European countries through vaccination before previous waves. Case totals may no longer reliably reflect the rate of spread, given the total number of cases, meaning that test positivity may now be a better guide — and that picture isn’t necessarily as encouraging. Especially in South Africa, testing may also be missing a large share of asymptomatic or mild cases. But the data we do have there shows an Omicron peak no higher than previous waves and notably more compressed as well — sketching a curve, if it continues its descent, too shallow to allow half the country’s population to be infected.

Even in those countries and regions where cases are still growing, it seems that the astronomical rate of initial spread — where cases were doubling every couple of days and estimates of the “reproduction number,” or Rt, suggested that each new infection was producing as many as six more — was short-lived. If the U.S follows those curves, it may see an Omicron peak as soon as mid-January, perhaps sooner — so fast that the 500 million free rapid tests the Biden administration mobilized in response to Omicron won’t even be available to order yet. We can’t take any of this to the bank, but we should probably be keeping as close an eye on case growth abroad and in the U.S. as we are on evolving estimates of severity.

What could explain rapid turns like that already taken by South Africa? And how confident should we be in projecting that it is replicated in the U.K. and the U.S., for instance, or countries with much different immunological profiles? It’s not entirely clear, but for once in the pandemic, the mysterious, animal spirits of disease spread may be working in our favor — in some cases seeming to reduce that initial reproduction number as much as four-fold within a week or two and without anything like the heavy-handed shifts in public policy or panicked disruptions to social behavior that have, in the past, tended to produce smaller reductions in rate of growth.

And a few particular hypothetical possibilities hover on the horizon, probably none a total explanation, and perhaps some not contributing any effect at all — only time and much deeper analysis will tell. If it turns out that Omicron has a much shorter generation time (meaning not that each case infects more additional people but that the infection turnover happens more quickly), that might mean that the initial reproduction number was not as high as originally thought — and can help explain rapid turns for the better as well. If it proves to be the case that Omicron continues to spread most dramatically among the vaccinated and already infected, with little or no advantage among the unvaccinated, it could mean both that the population susceptible to rapid growth was at least somewhat limited — and that testing may be missing a higher percentage of cases than in previous waves because prior immunity reduces severity and symptomatic disease, even if it isn’t very effective at reducing spread. It is also possible that the fast peak in South Africa, faster than those we may be seeing now across Europe, could imply a meaningful difference in susceptibility to infection between the vaccinated and the previously sick, since most of South Africa’s “immune wall” was built in previous waves, whereas most of Europe’s was built through vaccination — though seasonal effects could well be playing a significant role there as well (South Africa is in summer right now). And while we should all be rooting for a fast crest and decline, there is also the possibility that Omicron is not overwhelming the global pandemic so much as unfolding beside it in parallel — offering perhaps little additional immune protection against other variants, including future ones, and possibly leaving an ongoing Delta wave almost intact once, or if, Omicron subsides.

The next few weeks may well show a divergence between the U.S. and Europe, on the one hand, and South Africa and Botswana, on the other — cases may continue to grow rapidly across this country even as they continue to fall in Gauteng. And the memory of the Delta wave — far more brutal in the U.S. than in Europe — should remind us that even if Omicron does crash soon, it may have a much heavier footfall in America. But if Omicron does take a quick turn here, and the U.S. records something in the range of 10 million cases of the new variant, rather than something in the range of 160 or even 300 million, that is much, much better news than even a large reduction in severity. As Ed Yong wrote recently, it has been perhaps the country’s biggest moral political failing in the pandemic that it has chosen again and again to see the challenge in terms of individual risk rather than social or collective burden. It may be a sign of the same failing that we are still so focused on questions of severity and much less focused on the matter of spread.

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The Best News About Omicron May Not Be About Severity