Previous waves have been 'relatively small' – why modellers are worried about the Indian variant

·6-min read
A public health digital board warns the public of a Covid-19 variant of concern affecting the community in Bolton, northwest England  - AFP
A public health digital board warns the public of a Covid-19 variant of concern affecting the community in Bolton, northwest England - AFP

Here’s the thing about a virus that rises and falls exponentially: the middle path is perilously narrow. Make the right call and you’ll be skiing down a steep sunny slope, your friends in the bar below cheering every turn. But step the other way and you’ll tumble down an icy escarpment of granite shards and boulders just as fast.

The assessment published by Sage on Friday as regards the Indian variant will have come as cold comfort to the Prime Minister, hence his evident caution at the podium in Downing Street after reading it. It told him he was standing atop a precipice but not which way he should turn. The data he really needs remains obscured by a thick and disorienting fog.

Mr Johnson’s position is made all the more difficult by the politics and psychology of it all. Huge progress has been made battling back the virus over recent months, so there is all the more to lose. On the other hand, there’s the vaccine, our avalanche airbag – not to mention the country’s now awesome testing capacity and its ability to track and trace.

“Just go for it, old boy – we’re well protected”, is the shout from the shuttered businesses, lockdown weary parents, the unemployed and assorted thrill-seekers in the valley below.

So what does the data that are available tell us? How worried should we really be about the Indian variant interrupting our path back to something resembling normality?

A few things are clear. The variant is here and it sits in three broad areas of the country – London, the Midlands and the North West. Total numbers have more than doubled in each of the past two weeks but it is climbing from a very low base, with just 1,313 cases so far detected in total.

In a small number of inner city districts there are both rising incidence rates and a high proportion of cases, suggesting community spread. “This is most pronounced in London and the North West”, says Public Health England.

Three key characteristics of the variant will ultimately determine the threat it poses: the speed at which it transmits; its ability to evade the immunity given by prior infection or vaccination; and the severity of the illness it causes.

It has been classed a “variant of concern” because it ticks the first box – scientists are now all but certain it has some transmission advantage. There is also some laboratory evidence to suggest it may dilute immunity but not catastrophically so. Full vaccination (two doses) will provide protection against severe disease if the experience of healthcare workers in India is anything to go by, say experts. Finally, on the variant’s virulence – its lethality compared to existing stains – there is no evidence so far to cause concern.

So why worry about the speed the variant travels when an estimated 69 per cent of the population now have Covid antibodies, rising to over 90 per cent among the most vulnerable? What damage could it possibly do given those levels of population immunity?

It is this question that vexes almost everyone but the answer comes down to simple, if counterintuitive, maths. Professor Adam Kucharski, one of the Sage modellers from the London School of Hygiene and Tropical medicine, explains it like this: “The issue is that many people have a mental image that we’ve [already] had the biggest possible epidemic waves, whereas we’ve actually had ones that are relatively small compared to what could have happened without control measures in place.

“Because of these controls, only a fraction of the people who could have got infected in the past year or so have been infected, so they’re still out there.

“Of course, for many of these people vaccines have now decreased their risk substantially. But a very large number of infections that come with a very small individual level of risk can produce a similar outcome to a smaller epidemic that carries a larger individual level of risk”.

And this is exactly what the Sage modelling shows (see charts below). If the new variant is just 10 to 20 per cent more transmissible we will only see a mild bump in new hospitalisations. But if it’s 30 to 50 per cent more transmissible the numbers of infections grow so large that hospitalisations quickly rocket beyond the NHS’s capacity to cope.

“At this point in the vaccine rollout, there are still too few adults vaccinated to prevent a significant resurgence that ultimately could put unsustainable pressure on the NHS, without non-pharmaceutical interventions", says the Sage briefing to the Prime Minister.

“If [the Indian variant] does have such a large transmission advantage, it is a realistic possibility that progressing with all roadmap steps would lead to a substantial resurgence of hospitalisations”.

So the burning question then becomes, how much more transmissible is the Indian variant?

Here, unfortunately, the fog becomes as thick as an Alpine soup. Based on its initial calculations, Sage says that there is a “realistic possibility” (see scale below) that it could be 50 percent more transmissible.

But case numbers are currently too few and too diverse to know that today with anything approaching certainty. Secondary infections in imported cases are higher than in recorded domestic cases, for example. This suggests sociological factors may play as much a part in the speed at which the variant spreads as its biology. Only time - a week or two more - will tell.

What Sage is more solid on are the klaxons to listen out for. It lists four of them, and warns they could sound “extremely quickly, potentially even within days”. They are: any “emerging evidence of vaccine escape”, such as variant infections in the vaccinated; a “rapid increase in hospitalisations” in affected areas; new hotspots emerging which are not linked to travel or existing clusters; and cases in the North West doubling again in under a week.

The Scottish First Minister Nicola Sturgeon and her Welsh counterpart Mark Drakeford occupy the same fog-bound ledge as the Prime Minister. And increasing the stakes for all is the next stage of reopening, which allows indoor mixing to start again in pubs, bars, restaurants and homes to varying degrees in all three nations from tomorrow, Monday.

On Friday, Mr Drakeford said he had “paused” plans to allow smaller events to reopen in Wales, while Ms Sturgeon said tougher restrictions would remain in place in Glasgow and Moray for a further week. Only Mr Johnson, who is faced with a much bigger and less cohesive population, stamped his boots into his skis and prepared to set off.

“I have to level with you that this could be a serious disruption to our progress and could make it more difficult to move to step four in June,” he warned of the variant. “I must again stress we will do whatever it takes to keep the public safe”.

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