From a famous mathematician.

Your chances of sharing a space with someone who is Covid positive, based on a given prevalence of Covid in the local population and a given number of people in the space:

So (using the arrow as an example), when 7% of the local population have the virus, if you share a space (supermarket, pub, restaurant, SA taxi) with about 20 people, there’s a 50% chance that at least one of them will be Covid positive.

For those that are interested in the maths, I’ve calculated the probability of at least one person in the room having Covid as 1-(1-p/200)^N Where p is the population prevalence as a percentage and N is the number of other people in the space.

In the UK at the moment, prevalence is somewhere around 4%: 1 in 25 people are positive. And that means that if you are packed tightly in any given space together with any more than about 90 people, there will be at least one of you who is positive…

Oh. Oh dear.

And yet there are still no precautions of any sort being put in place. Quite bizarre

Of course there are a few terms and conditions to Kit’s graph.

Some people might isolate if they are Covid positive. Some people might be Covid positive but asymptomatic. Some people might not have bothered getting tested, so they won’t know if they’re positive or not. All of these might affect the assumed prevalence.

And then, of course, just because you share a space with someone who is Covid positive doesn’t mean you’ll get infected (although, see headline above). That will depend on the size of the space (and therefore your proximity), the amount of time you and they spend in the space, the ventilation in the space and – of course – whether either (or both) of you is wearing a mask.

But it’s a super useful tool for just showing how likely any of us is to be sitting next door to someone who is positive. Lovely stats. Only mildly scary.