In early September, outbreak modelling for the government’s Sage advisers showed Covid hospitalisations had the potential to soar. If people rushed back to work and resumed all the socialising they had put on hold, the number of daily admissions in England could peak at 7,000 within six weeks. It was, in effect, a worst-case scenario, barring a dramatic waning of immunity or a troublesome new variant.
The optimistic scenario looked very different. Assuming a more gradual return to normality, the modelling had daily Covid hospitalisations rising slowly and slightly, topping out at nearly 2,000, before falling again in November. Now, even that looks overly gloomy. Over the past fortnight, hospitalisations have fallen in England, even as schools and offices reopened.
Mismatches between the modelling and the true course of the epidemic have caused confusion throughout the Covid crisis. The models are not predictions of what will happen. They are what the computers churn out when presented with a “what if?”. In this case, what if R (the reproduction number of the epidemic) reaches 1.1? And what if – as Jonathan Van-Tam, the deputy chief medical officer for England, would say – people “rip the pants off it” and push R to 1.5? That would mean, on average, every two people infected go on to infect three more.
R, or the 'effective reproduction number', is a way of rating a disease’s ability to spread. It’s the average number of people on to whom one infected person will pass the virus. For an R of anything above 1, an epidemic will grow exponentially. Anything below 1 and an outbreak will fizzle out – eventually.
At the start of the coronavirus pandemic, the estimated R for coronavirus was between 2 and 3 – higher than the value for seasonal flu, but lower than for measles. That means each person would pass it on to between two and three people on average, before either recovering or dying, and each of those people would pass it on to a further two to three others, causing the total number of cases to snowball over time.
The reproduction number is not fixed, though. It depends on the biology of the virus; people's behaviour, such as social distancing; and a population’s immunity. A country may see regional variations in its R number, depending on local factors like population density and transport patterns.
Hannah Devlin Science correspondent
Sage expected hospitalisations in England to peak somewhere near the lower range, namely 2,000 a day, but no sooner was the modelling complete than hospitalisations began to fall. The decline was unexpected. What it suggests is that – for now – the effect of unlocking on fuelling the epidemic is more than offset by the combination of people’s behaviour and immunity, whether from vaccination or infection.
“Those are two very powerful forces. Each by itself is perfectly capable of making the number of cases or hospitalisations go up or down, and they are basically fighting each other right now,” said Mark Woolhouse, professor of infectious disease epidemiology at Edinburgh University.
On Friday, the Office for National Statistics reported a fall in infection rates in England for the second week running, with one in 90 now estimated to test positive for Covid. Elsewhere in the UK, rates remain stable but high.
According to Prof Graham Medley, chair of the Sage modelling subgroup, Spi-M, while infections and admissions have drifted down in the past couple of weeks, little has changed over the larger timeframe of the past 10 weeks. “This is unexpected,” he said. “There must be a balance between the increasing immunity from infection and vaccination, and the amount of contact, but how they exactly balance to keep R roughly at 1 is a bit of a mystery.”
It may be that vaccines are more effective at preventing transmission than studies – often based on symptomatic patients – suggest. If that is the case, Woolhouse said, immunity may be playing a larger role in suppressing the epidemic than thought. After a sharp rise in Scotland, cases appear to be falling back down, without any obvious change in behaviour, he added. “It’s a watershed moment. This is the first time in the history of the UK’s epidemic that we’ve had a sustained decline in cases in the absence of a lockdown or not far short of it,” he said. “We’ve never seen that before, so clearly something is fundamentally different, and the fundamental difference for me is the buildup of herd immunity.”
That would be excellent news, particularly if the rest of the UK follows suit. On Friday, the R number for England was revised to 0.8 to 1, with the number of new infections estimated to be shrinking at 1% to 3% a day. The difficulty is that, with a lot of virus still around, a manageable situation could become challenging very fast. “If there is an uptick then we need to react to that quickly. If this does go wrong, the NHS will be in trouble very quickly,” Woolhouse warned.
As Medley pointed out, the country has not rushed back to “life-before-Covid”. What happens next is still as murky as ever. “We are still a long way from normal levels of contact, so there is still the possibility of an increase in transmission and hospitalisations, but the past couple of months gives a lot of hope,” he said.