Are Covid cases doubling or not? And if so, in what time frame? If you listened to Boris Johnson and chief scientific adviser Patrick Vallance, you’d be forgiven for being confused. The Prime Minister said this week:
‘The chief medical officer and chief scientific adviser warned that the doubling rate for new cases could be between seven and 20 days.’
In fact, Vallance had said this:
‘At the moment we think that the epidemic is doubling roughly every seven days.” And added, “It could be a little bit longer – maybe a little bit shorter – but let’s say roughly every seven days.’
So what’s going on? In order to understand what is happening, it’s important to follow four steps when looking at Covid case numbers as reported each day:
1. Work out what time frame you want to look at: you can either choose the date the specimen was taken (‘specimen date’) or the date the results were reported.
It is best here to choose the specimen date, not the reporting date. Why? Because the actual test could have occurred some time before (symptoms could be even earlier) as opposed to the date of reporting, which can fluctuate due to delays in reporting. This is a similar problem to delays in the reporting of deaths that led to changes in the way they are reported. And the results (as you can see below) vary considerably:
Cases by Specimen date
For September 2020, using the government’s published data, you will see that in England the cases based on the date the specimen was taken were 2,614 on 2 September and increased by 1,098 (42 per cent) to 3,712 over a two-week period by the 16 September. (By this coming Monday, the case numbers will be sufficiently stable to further assess the changes in case numbers for the 23 September.
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