More ambiguously, last week there were about 6,000 hospital admissions with Covid in South Africa, all of which were presumably Omicron since the variant is now dominant there. That’s well below the July peak of about 15,000 a week, although there’s a lag between infection and hospitalisation.

This sort of news has led to people thinking that it’s much less deadly than Delta. As the Omicron wave looms over Britain, that sort of hope keeps us sane.

But how much can we trust that hope? The Prime Minister has promised to ramp up the booster programme, saying that everybody over 18 would be vaccinated by 31 December – about a million people a day. Can that really be possible? And if it were, would that be enough, or are we going to need something more than the “Plan B” restrictions that are already in place.

First, we need to know if it really is less deadly than the other variants. It’s not as clear-cut as all that. South Africa has only fully vaccinated about a quarter of its population, but it has had three large waves of infection: very probably, by now, most of the population has had the disease. So it’s not possible to directly compare the Omicron wave with the Delta wave: Omicron is working its way through a population with much higher immunity.

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The trouble is, that won’t be true in the UK. When Delta came here, we were already highly vaccinated: almost every adult had had at least one jab. At least some of the reasons why Omicron appears to be less of a threat to South Africans don’t apply here. We’re also a much older population.

Closer to home, Danish data looking at 2,417 cases of Omicron found only 19 people had been admitted to hospital. But again, it’s hard to know what to make of it. The patients involved were mainly young – the median age is about 30. And given an average lag of about a week between infection and hospitalisation, and since about 2,000 of the cases were detected in the last week, it might not be as promising as it sounds on the surface.

The most recent UK Health Security Agency technical briefing similarly finds that “None of the cases is known to have been hospitalised or died”. But their data only goes to 6 December. By 6 December, there had only been 260 cases of Omicron in the country, most of which had been detected in the previous few days. Since then, there have been at least 10 hospitalisations and at least one death linked to the new variant. Things may become clearer in the coming weeks.

It’s perfectly possible that Omicron is less dangerous than Delta to a person with prior immunity. But we can’t be sure yet. And we should also remember that while deadliness — virulence — is important, it’s not as important as transmissibility.

Imagine the following scenario. You have a disease with an R of four — that is, on average, each person with the disease gives it to four more people. And it also has an infection fatality rate (IFR) of 4%, so it kills one person in every 25 that it infects. (These are made-up numbers for illustration.)

You follow that disease through 10 generations. The first person would infect four more; those four would infect a further 16, and so on. By the time you reached the 10th generation, you would have about a million new cases – and adding all the generations together, you’d get a total of 1.4 million cases. Of those 1.4 million, about 56,000 would die.

Imagine that a new variant arrived which was more deadly: which kills 5% of victims. It would be worse. You’d expect about 70,000 deaths, 25% more than before.

But now imagine that a new variant arrived which was more transmissible, by the same margin. Each person infects five people, on average. Suddenly the first one infects five, those five infect 25, those 25 infect 125. By the time you reach the 10th generation, it’s almost 10 million new cases, and about 12 million in total. Even if it still only had an IFR of 4%, it would kill about 480,000 people – nine times as many as the original.

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So even if Omicron is less deadly, if it’s very good at spreading through the population, it could quickly overwhelm the health service just through sheer weight of numbers.

The question, then, is: is it more transmissible? There was much excitement over the weekend, when the case numbers appeared to be slowing there — after zooming up to 19,000 a day, they dropped back to 17,000, and people started to wonder whether it had peaked. But then the South African health service released a new tranche of data on Sunday, with another 37,000 cases — the apparent slowdown appears to have been a reporting backlog.

Even taking that into account, Omicron isn’t doing quite the mad upwards dash that it was in late November. But it’s still escalating, in a presumably fairly immune population. And it seems to be spreading even faster in the UK: Paul Hunter, a professor of medicine at the University of East Anglia, told the Science Media Centre that “In South Africa the latest R estimate is about 2.2. For the UK the estimate is 3.7 which is doubling every 2 to 3 days.” The UK HSA technical report suggests (p19) that it spreads around twice as easily as Delta in the UK population. If that’s true, it would easily overwhelm any gains from reduced virulence, even if they’re real.

So what does it all mean? Over the weekend, the London School of Hygiene and Tropical Medicine released a model of how Omicron might spread. It included scenarios with different assumptions about how good the new variant is at evading immunity, and about how good the boosters are at building that immunity back up. It also assumed that we’d keep the Plan B advice in place (masks in some public spaces, people working from home if they can, some vaccine-status checking) and that the NHS would administer about 500,000 new booster jabs a day.

The most likely scenario is that the variant is pretty good at evading immunity, but that boosters work pretty well. Under that scenario, the LSHTM model’s projections look fairly bleak: a peak of about 6,000 hospital admissions a day, compared to about 4,000 a day at the January 2021 peak. And they think between about 40,000 and 50,000 people would die of Covid between 1 December and 30 April.

They say in the model that they have assumed Omicron is no less deadly than Delta, given prior immunity. They might be wrong about that, but as I’ve shown, we can’t be sure. It’s certainly very possible that we’ll be looking at another wave of deaths comparable to the first or second one.

Will Johnson’s booster boosterism make much of a difference? I doubt it. It takes at least a week for boosters to have a significant effect on your immunity, and for some people more like two. Even if we started vaccinating a million people a day from tomorrow, the impact of that wouldn’t be felt until around Christmas. We’re not going to start vaccinating a million people tomorrow: we’ve been averaging 400,000 a day recently, fewer than the LSHTM model assumes, and the NHS thinks that we’ll still be boosting well into January and February and we’ll only have offered everyone a jab by the new year. Hopefully it’ll ramp up quickly, but if we’re relying on boosters, then most of the January caseload will be baked in already.

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And here’s the really worrying thing. Sajid Javid, the health secretary, told MPs last night that 200,000 people have been infected by Omicron yesterday alone. This was such a huge claim that people assumed he’d got mixed up: perhaps he meant 200,000 people are infected? Or have been? But apparently not. There were about 50,000 cases reported in the UK on 10 December. The ONS infection survey suggests that we only detect 40% of them: that is, there were really 125,000. On the 10th, 19% of cases were Omicron. That is about 25,000 cases. But cases detected on the 10th were probably infected on the 7th, six days ago. And if it’s doubling every two days, then that 25,000 has had time to become 200,000.

For reference, that’s about the number of daily cases that we had in early January 2021, the peak of the second wave, by which time we’d already been in full lockdown for a month. We are not in lockdown now, and several more doublings are probably already locked in. If Javid is correct, then Omicron had better be much less deadly, or there could be real trouble ahead. We’ll know if he’s right in about three days. 

So should we be talking about restrictions?

The restrictions we put in place for the first and second waves were intended to avoid something far worse. The 16 March 2020 paper by Imperial College London predicted 500,000 deaths if no measures were taken. That was what sent us into lockdown. The actual death toll of 40,000 or so in the first wave was a fraction of what it could have been.

But we are nowhere near where we were early last year. We’re now talking about something that is on the level of a very bad flu season. Full-on March 2020 lockdown is probably an overreaction.

But the LSHTM model thinks that we could prevent about two-thirds of those deaths and hospitalisations by going to full lockdown,[1. It actually only models the most optimistic and most pessimistic scenarios under the various restrictions. But in both of those scenarios, about two-thirds of deaths and hospitalisations are prevented under strictest lockdown, and about a third under the less stringent Stage 2 restrictions. I have assumed that the intermediate scenarios see roughly similar numbers.] and about one-third by introducing, right now, some less strict measures, equivalent to Stage 2 of the “roadmap out of lockdown” from spring: the biggest part of that was probably that only two households or six individuals could meet indoors.

It is a non-trivial intervention that could slow the course of Omicron; it might buy some time for the sped-up booster programme; and it would mean we wouldn’t have to cross our fingers and hope that Omicron is less deadly than Delta. Obviously politicians will be reluctant to ruin another Christmas. But relying on boosters alone is a big risk, especially since no one but Johnson seems to think we can hit the target.

Omicron has been detected — and may have first arisen — in Gauteng province, South Africa. There have only been a few hundred cases detected, so it’s hard to say much about it with any real certainty. But it appears to be more transmissible and there are concerns that immunity, whether from vaccination or prior infection, will be less effective against it. There have been, as I write, three cases detected in the UK, and there are real (and justified) fears that we are heading once again into a lockdown Christmas. 

But nonetheless, we have been very lucky.

We mustn’t forget that the virus is constantly mutating. Every time it copies itself, there is a chance that it will make some minor error. Most of those mutations either have no effect or make it less effective at spreading. But every so often one will improve it: perhaps make it harder for the immune system to see, or make the virus quicker at copying itself.

The other new variants, notably Alpha and Delta, had several mutations, but Omicron has dozens: around 50. And 10 of them are on the bits of the “spike” protein on the virus’s surface which bind with our cells — the “receptor binding domains”. Sir Jeremy Farrar, the director of the Wellcome Trust and former member of SAGE, tells me that these are exactly the kind of mutations which virologists have been expecting and dreading: “It’s biologically plausible that they will drive higher transmission. They’ve been identified before as ones we should be really careful about.”

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So far, it doesn’t sound very lucky. But in one mutation, Omicron has been kind to us. Like Alpha before it, it has a particular mutation, 69-70del, two missing RNA letters in its genome — part of the “S gene” which codes for the spike protein. By happy chance, those missing letters are at one of three places that many PCR tests look at.

What it means is that if a PCR test comes back positive on two out of its three areas — if it has an “S-gene dropout” — then you can be pretty sure you’ve got one of the variants. It means that Alpha last year and now Omicron can be tracked pretty effectively, just with PCR testing: even without sequencing the genomes, you can see where these S-gene dropout results are happening and it’ll give you a good idea of how the variant is spreading.

“It’s a complete gift that the S-gene dropout hits this variant,” says Ewan Birney, the deputy director-general of the European Molecular Biology Laboratory. If it didn’t, we would have much less of an idea where Omicron is. (We’ve actually been lucky twice. If Delta hadn’t replaced Alpha, then the S-gene dropout wouldn’t have told us anything, because both the circulating variants would have had it.)

So far, so good. But is the new variant more transmissible and/or more able to bypass our immunity, whether from vaccines or infection? And is it more or less dangerous to people who catch it?

These are hard questions to answer, because at the moment, we only have data from South Africa, and South Africa’s population is tricky to compare with ours. It’s younger, for a start — the median South African is 26-years old, while the median Brit is 40. It’s also much less vaccinated: somewhere around 75% of Britons have been double-jabbed, while barely a quarter have in South Africa (which is, to my surprise, more to do with uptake than supply). It has also had far fewer confirmed cases per capita.

That said, we can make some guesses. When a new variant emerges, it’s difficult to tell whether it has spread because it is better at spreading, or because it happens to have arisen in a convenient place. Imagine a virus which happened to mutate into a new variant in someone who was about to go to a major conference. It might infect hundreds of people and show up all over the country, but it wouldn’t be because it was any more transmissible — it was just that it got lucky.

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With Omicron, this seems unlikely to be the case. Thanks to the S-gene dropout, we can see that it’s spreading faster than you’d expect. Farrar says that “there has been a clear upswing of transmission in Gauteng, which seems to be driven by the variant”. “It probably can outcompete Delta,” says Babak Javid, an immunology lab director at the University of California San Francisco, “although by how much is still up in the air.” 

Is that because it’s better at spreading in general, or because it can evade immunity? So far, we don’t have the data to say for sure. Soon, we’ll start to get results from laboratory studies, and later from epidemiological observations, but Rupert Beale, leader of the Cell Biology of Infection lab at the Crick Institute, thinks it’s “a few weeks before we get useful information, and the other side of Christmas before we know anything definitive”.

But on the plus side, antibodies aren’t the only part of our immune system. We also have T-cells, which are less easily fooled by mutations. To oversimplify, antibodies stop you getting infected in the first place, but T-cells stop you from getting very ill. So Javid and Birney think it is plausible that Omicron will spread quite easily among vaccinated and/or previously infected people, but will be less likely to kill them or make them severely ill.

There has also been speculation that Omicron is inherently less deadly to people with no pre-existing immunity. That’s not impossible, but everyone I spoke to said that any definitive conclusions are premature. “The speculation is extremely uninformed,” says Beale. “We just don’t know until it gets into a more vulnerable population, which sadly it seems it’s likely to. We can hope it’s less pathogenic, but it’s foolish to make that assumption.”

So it might be better at spreading, and able to escape our immunity to at least some degree; and while it’s theoretically possible it might be less deadly, that’s certainly not a proposition we want to put any weight on. The question is: what should we do, and are we facing a second bleak Covid winter?

The pandemic should have taught us that acting early is usually better than acting late, and that preparing for worst-case scenarios is important. It’s like chess, says Beale: “You play your moves in the most flexible order.” If we assume it’ll all be OK, we might be right, but if we’re wrong, we’ll end up with Omicron spreading all over the place. Whereas if we assume it’ll be pretty bad, we can always open up again later on.

I’ve seen a lot of people, some of them very sensible, suggesting that there’s no point closing borders to South Africa because variants will get through anyway, and that it’s just punishing countries for doing good surveillance. But I think they are wrong. 

Yes, the variant will get through, if it hasn’t already. Farrar points out that it was first identified on 11 November, and its sequence suggests that it had already been circulating for some time before then. “I think it may already be on every continent,” he says. But that doesn’t mean that there’s no point in trying to reduce its spread. “Border closures might buy you a little time.” Birney agrees: “Shutting the borders until we understand it, minimising the seeding events, is a good rational move.”

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It’s true that there’s a risk of punishing countries for responsible surveillance, but I don’t see what the alternative is — pretending that they don’t have lots of Omicron circulating? Perhaps the thing to do is also put restrictions on countries with poor surveillance.

Because “buying a little time” might be important. If nothing else, it gives us time to understand it: if Omicron is highly virulent and vaccine-resistant, then our best response will be rather different to what it would be if it isn’t. It also gives us a chance to get more boosters in arms and to stockpile antiviral drugs. 

All the same arguments apply to other non-pharmaceutical interventions, such as introducing vaccine passports or asking people to work from home if possible. But there are other things Britain can do, too. 

One thing is to hugely increase our vaccine manufacturing capacity — something which would be good for us and for the world. Rolling out children’s vaccines and booster jabs faster wouldn’t hurt. One scientist I spoke to suggested making antiviral meds available by post when people get a positive result: “They need to be given quickly to have a good effect, so you can’t rely on GP appointments. You need to link them to Track & Trace”. Cheaper, quicker PCR testing at airports would help.”

But while those measures are important, they’re not addressing the root of the issue. For Farrar, the emergence of Omicron is just another reminder that we need to vaccinate the world, not just the rich world. “There’s been political drift for months now, in terms of access to vaccines,” he said. “The rich world thought it was over, so they’ve been kicking it into the long grass.” Variants like Omicron will continue to arise until the world has a decent level of immunity.

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We’ve been lucky, with Omicron. But it may well still end up becoming the dominant strain, just as Delta did before it. If it doesn’t, then another one will come along. And it will be “incredibly disruptive”, says Farrar: “Everyone’s putting restrictions in.” Certainly, to avoid another lockdown Christmas, we need to act now.

But if we want to be sure we don’t have another one next year, we need to get this pandemic behind us — and that means billions more doses of vaccine, distributed around the world. We won’t always be so lucky.