Is Omicron replacing Delta or spreading alongside it?

AP Photo/Angie Wang

That’s one of the most consequential questions of the pandemic right now. If Omicron produces milder symptoms than Delta, as appears increasingly likely, and immunity conferred by an Omicron infection protects the infected from Delta, then the new variant is a blessing in disguise. It’s essentially vaccinating people against a more dangerous strain.

But if an Omicron infection doesn’t protect you from Delta then we’re in a grim situation. We’ll have two pandemics raging side by side, one more dangerous than the other. Some people will be infected by both variants. And even the milder strain will make a meaningful number severely ill by dint of the sheer size of the population it seems destined to infect.

News came last night that Omicron accounted for 73 percent of all cases in the U.S. last week, a phenomenal degree of spread relative to a variant like Delta that’s itself highly contagious. The temptation upon hearing that is to conclude that the new variant is driving the old one to extinction, but that’s not necessarily so. If there were 50,000 cases of Delta and no cases of Omicron yesterday and there are 60,000 cases of Delta and 600,000 of Omicron today, the share of Delta cases would have gone from 100 percent to less than 10 percent overnight — and yet there’d be more Delta circulating in the population today than yesterday.

Scott Gottlieb is watching the UK with some anxiety:

β€œI anticipate that over time that Delta will be crowded out by Omicron,” Rochelle Walensky said yesterday. Let’s hope so. Because the conventional wisdom that’s congealing in America is that Omicron is a nothingburger so we should drop all personal precautions. That could work out fine if the conventional wisdom is right but only if Omicron really is crowding out Delta. If people are letting down their guards while Delta is surging alongside the “mild” variant, they’re at risk of running into the more dangerous strain.

Even if Omicron does confer some immunity against Delta, the fact that cases appear to be falling already in South Africa suggests it might not last long. We’re in a strange spot, in other words, where we’re rooting for the new variant to be different enough from Delta that its symptoms are much milder but not so different that getting infected by Omicron does nothing to prepare your immune system for Delta. Former Biden COVID czar Andy Slavitt captured the strangeness:

A super-infectious variant that kills hardly anyone and drives the more dangerous strains to extinction is wonderful news. But it needs to do both, not one or the other.

The only certainty is that Omicron is so fantastically transmissible that even seasoned pros like Tom Frieden are in awe of what they’re seeing. Frieden is a former head of the CDC and not known for breathless hyperbole about COVID but he’s in near-disbelief at the speed with which it’s spreading:

I listened to a recent interview with virologist Trevor Bedford in which he said he doesn’t think Omicron is inherently more infectious than Delta. They both spread extremely quickly but in a population in which no one had immunity to either strain Bedford expects Delta would win the race. (Even with Omicron appearing to have a shorter incubation period than Delta, though?) The reason Omicron is moving at light speed relative to Delta at the moment is that the immunity we’ve gained since March 2020 works reasonably well to contain Delta. That variant “looks” sufficiently like the original Wuhan virus that many people who’ve been vaccinated or infected by a prior strain can fend off Delta without being infected (especially if their immunity hasn’t waned yet), limiting its ability to spread.

Omicron “looks” different enough from the original strain that it’s able to infect many more people with prior immunity than Delta did, though. Everyone’s a vector of transmission. A variant with hardly any limitations on its ability to spread will obviously spread much more quickly than one that’s constrained. But that goes to show why Gottlieb is worried: If Omicron can blaze through walls of immunity that are capable of extinguishing Delta, maybe the two variants are dissimilar enough that an Omicron infection won’t protect you from Delta.

But we shouldn’t get ahead of ourselves. The chief concern at the moment is whether Omicron itself is truly mild, given how many people it’s going to infect. Early indications remain encouraging:

The U.S. must have had several hundred thousand confirmed cases of Omicron already. As of today, there’s been exactly one fatality and that person was unvaccinated and had underlying conditions. (He also had natural immunity, though, having been infected with COVID before.) It takes a few weeks before the average fatal case kills someone so we can’t guess yet at what sort of death toll there might be, but no country that’s been exposed to Omicron over the past month is seeing a huge death wave at the moment.

In any case, we should have the answer to the question posed in the headline in a few weeks. If Omicron is milder than Delta and it’s also conferring immunity to Delta as it spreads, the death toll in the U.S. should begin to drop in January even as cases skyrocket. People who otherwise would have been infected by Delta and died from it will be “vaccinated” by a case of Omicron beforehand, ultimately sparing them. If the death toll doesn’t drop, that’s a clue that Delta is still killing people despite Omicron’s spread — or that Omicron itself is fatal in some cases. Or both, of course.