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March 5, 2020 - 7:00am
I had a notable experience on the Tube the other day. It was lunchtime, not very busy. A man, standing at one end, sneezed. A woman near him immediately got up and walked away.
My immediate assumption was that she was worried about coronavirus. I may have been wrong, but let’s say I’m right. I wondered: very roughly, what are the chances of that man having had coronavirus?
(Note: this isn’t meant to be medical advice, or to tell you that it is not rational to avoid people who are sneezing. It’s just in case a ballpark estimate might be helpful.)
I’m writing just after 32 new cases were confirmed in the UK, for a total of 85. That’s likely to be a major underestimate. They have tested about 16,000 people, so about 0.5% of those tested were positive; we could scale that up, meaning 0.5% of the UK population of 70 million – about 350,000 – would have it. But that would obviously be a huge overestimate, because they’re testing the most at-risk people. So we have the bounds of the problem: no less than 85, but no more than 350,000.
I’m going to arbitrarily assume that there are about 100 times as many cases as we’ve found so far. That’s two orders of magnitude higher than the confirmed cases, but two lower than the upper bound.
That would mean 8,500 people with coronavirus in the UK. Let’s assume they sneeze as often as people with colds and flu, and healthy people never sneeze. The average adult gets 2-3 colds a year and they last a week to 10 days. Let’s take the lower estimates and say 14 days with colds a year, so 4% of population has a cold at any one time. (We’ll ignore flu because it’s less common.)
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