Is R number balanced on knife edge?
There have been warnings coronavirus may be starting to spread again in the north-west and the south-west of England.
Some scientists say the R number is creeping up across the country and may have surpassed one – the point at which the epidemic takes off again – in these regions.
Tameside Council has “strongly” advised schools not to reopen today.
The government insists the number is not above one anywhere in the country.
So, what is going on? How worried should we be? And what does it mean for lifting lockdown?
Quickly, what is the R number again?
It is the number of people each infected person, on average, passes the virus onto.
If the R is three, then 10 people would infect 30 others. But if R is 0.5, those 10 people would infect only five.
An R of one is the crucial threshold. Anything lower and an epidemic is in decline, but if it goes higher then cases increase exponentially.
What are the scientists saying?
That R is increasing across England.
Their findings suggest we’re now balanced on the edge of a knife.
“Both the north-west and south-west are just above one, but all regions are straddling one,” said Dr Paul Birrell, one of the researchers.
But this is just one mathematical model and there is huge uncertainty in the figures. The PHE data gives a figure between 0.72 and 1.2 for London.
The London School of Hygiene and Tropical Medicine agrees the south-west may be above one, but paints a more optimistic view of the rest of England and their R number for the north-west is just 0.8.
“Overall, I would say we see R creeping up,” Dr Sebastien Funk, from the London School, told the BBC.
They also say the R number is 0.8 in Wales, Scotland and Northern Ireland.
Is this a catastrophe for lifting lockdown?
It would be easy to say yes, but the reality is confusing and it is important to understand what is driving any changes in R.
“It’s complicated, the reasons are many and difficult to untangle,” Dr Funk told me.
The most straightforward explanation is that the virus is genuinely spreading more. As the PHE report says: “We believe that this is probably due to increasing mobility and mixing between households and in public and workplace settings.”
But due to a counter-intuitive quirk of maths, lockdown can be a success, cases fall and R still goes up.
If cases collapse in the community – which appears to be happening – but come down more slowly in care homes and hospitals – which we also think is happening – then these areas will drag the R number back up, even though the situation is improving.
And when you get to low levels of infection, all the estimates become more uncertain and that too can make R appear higher.
“It’s a weird thing about R, it tells you the rate of change, and once all the way down, you can’t have R at a low level,” said Dr Funk.