The above graph is the COVID-19 epidemic curve for England, reconstructed by Imperial College’s REACT antibody survey by asking those who tested positive in an antibody test when their symptoms began. I’ve added the start dates for lockdowns in red and the end dates in blue.
It’s a very useful graph because it does not involve any PCR tests at all, only lateral flow immunoassay tests, self-administered at home. This means it does not suffer from the problem of detecting non-infectious virus as it is not detecting virus at all but antibodies. (Its specificity is reported as 98.6%, giving it a 1.4% background false positive rate, which the researchers adjust for.) This means, for example, that the epidemic decline is much faster than in the familiar “case” curves, and the curves are more symmetrical.
What does it show? Here’s what I take from it. You might see more.
Firstly, it provides further evidence that SARS-CoV-2 was circulating at low levels in England throughout December 2019 and to some degree also in November. This fits with widespread anecdotal evidence of people falling ill with Covid symptoms in December. It doesn’t fit with the original official timeline of an outbreak beginning in Wuhan in December.
Secondly, despite circulating widely during the winter of 2019-20, SARS-CoV-2 did not undergo fast spread in England until the end of February. Indeed, the winter of 2019-20 was the least deadly on record in terms of age-adjusted mortality, despite SARS-CoV-2 being around and infecting people.
Then, around February 25th 2020, it suddenly launches into a three-week long spike of extraordinary exponential growth. This abruptly comes to an end around March 17th, and after a short plateau till around March 21st it enters just as extreme a decline. This is all ahead of the first lockdown on March 23rd of course.
The mystery is: what happened on February 25th (or thereabouts – we don’t know whether Imperial’s assumptions about the incubation period are exactly right) to cause a virus that had been circulating for at least three months at a low level suddenly to go bang and spread like wildfire? It wasn’t panic – no one was panicking at the end of February. Mobility levels were still normal until around March 12th. There was nothing unusual about the weather. Suggestions on this welcome in the comments below.