Every story about the new variant is a “good news/bad news” thing and today’s is the supreme example.

One theory of why fewer people going to the hospital in South Africa during the Omicron wave than in previous waves is that it’s not so much that the virus is weaker as that the population is stronger. South Africans are younger on average than westerners are and a great majority of them have been infected before. They’ve built up a degree of natural immunity that may have left them well prepared for Omicron such that their younger, more robust immune systems are fighting it off. Whether the same will happen in the U.S. is anyone’s guess.

The other theory is that the new variant really is inherently mild. The many mutations that have made it more transmissible may have also made it less severe somehow. But … how? What’s happening inside the bodies of people infected with Omicron that’s different from what happens when they’re infected with Delta?

A study of infected respiratory tract tissue by the University of Hong Kong may have the answer. Omicron replicates at fantastic speed in an infected person — but not in the lungs, where it can do real damage. It’s happening in the bronchus, the tubes that connect the lungs to the windpipe. In the lungs, Omicron actually spreads less efficiently than the original Wuhan strain or Delta do.

That adds up with what’s happening in South Africa. Doctors there are seeing fewer cases of COVID pneumonia, a disease of the lungs, than during previous waves. Maybe Omicron really did mutate into something less intrinsically severe than prior strains, which would be wonderful news.

But don’t overlook the data on the left-hand part of the graph while you’re busy focused on the right-hand part. Omicron is less likely to impair the lungs, it seems, but it’s replicating within the bronchus at a phenomenal rate even relative to Delta. That also lines up with what’s happening in South Africa. If there are vastly more viral particles in the average infected person’s airway than there were with previous variants, it makes sense that Omicron would spread with lightning speed. If HKU is right, there must be substantially more virus being exhaled each time an infected person breathes than was true in prior waves.

Is a fantastically infectious virus that’s inherently less dangerous a good thing on balance? There’s no way to be sure yet.

The only thing I am sure of is that omicron will spread so quickly through the population, making it likely impossible to contain even with the most stringent measures & giving us very little time over the next few weeks. So get your vaccines & boosters! https://t.co/NUGSb1ndT7

— Muge Cevik (@mugecevik) December 15, 2021

Even if Omicron spreads less efficiently in the lungs, how quickly it’s quashed will also depend on how strong a patient’s immune system is. Older people may yet have difficulty with it.

Scientists are split on how dangerous the coming wave will be. One UK expert is optimistic: “My gut feeling is that omicron is very much like the sort of flu pandemic we planned for – a lot of sickness absence from work in a short period, which will create difficulties for public services and economic activity, but not of such a severity as to be a big problem for the NHS and the funeral business.” No one doubts that a truly gigantic wave of cases is coming, though. This language from the head of the UK’s Health Security Agency is striking:

Omicron is “the most significant threat” to public health in the U.K. since the start of the pandemic, authorities warned Wednesday, as the World Health Organization cautioned that the highly contagious Covid-19 variant was spreading at an unprecedented rate.

The number of Covid-19 infections over the coming days will be “quite staggering” compared to previous variants, the head of the U.K. Health Security Agency, Jenny Harries, said in a warning about the threat omicron posed.

What does Harries mean by “staggering”? Well, here’s the case curve in Norway at the moment:

And here it is in Denmark:

Both countries have higher vaccinations rates than the U.S. and UK. Both have populations of around 5.5 million yet are seeing case counts in the range of 5,000 to 7,000 per day lately. That would shake out to well over 300,000 cases per day in the U.S. if we experience the same sort of surge. And note well: Neither Norway nor Denmark has peaked yet. They’re still on the way up.

The silver lining in all this for optimists is that, hopefully, everyone who catches Omicron will be rendered immune from the seemingly more dangerous variant, Delta. But there’s no guarantee of that. A few days ago I flagged this analysis by Trevor Bedford, who speculated that cross-immunity between the two variants might be less than we had hoped. If Omicron is different enough from prior variants to infect people who’ve had the virus before then catching Omicron might not generate the antibodies you need to repel Delta. The CDC is also worried about that possibility, fearing a worst-case scenario this winter in which Delta, Omicron, and the flu all circulate side by side:

The worst-case scenario has spooked top health officials, who fear that a fresh wave, layered on top of delta and influenza cases in what one described as “a triple whammy,” could overwhelm health systems and devastate communities, particularly those with low vaccination rates.

“I’m a lot more alarmed. I’m worried,” said Marcus Plescia, chief medical officer for the Association of State and Territorial Health Officials, who participated in the call. The CDC, normally cautious in its messaging, told the public health officials that “we got to get people ready for this,” he said.

There have been many reassurances lately as the Omicron news has percolated that, although the new variant is more likely to punch through people’s immunity, we’re not “back at square one.” And that’s true, of course. We have a gigantic amount of population immunity now, both natural and vaccine-derived, whereas we had none in March 2020. And we have promising therapeutics to prevent severe disease, like Pfizer’s new wonder drug. We also have 22 months of expertise in treating COVID patients developed by the U.S. health-care system. The average person is much less likely to die from infection today than they were early last year.

But in terms of the phenomenal rate of spread of the new variant, the worry about the “curve” overwhelming hospitals, and the sense that there’s nothing one can do to avoid infection except to isolate, there is a bit of a March 2020 “back to square one” vibe among the expert class.

The obvious solution is boosters, which have proved effective against Omicron in every study to examine the matter so far. But there’s a catch there too: Boosters work partly because they’re given long after the prime doses of a vaccine are given. That delay gives the immune system time to develop a more mature response to the virus, which is then enhanced when it gets a third exposure via the booster. That’s why boosters are currently recommended only for people who got their second dose more than six months ago. But many Americans got their second dose more recently than that; if you boost them now, without giving their system time to develop that more mature immunity, you might not get much benefit. Scientists are worried about that too. We’ll see what the CDC ends up doing.