New Covid-19 cases in South Derbyshire
According to the Govt. website, five new cases were reported in South Derbyshire today (Wednesday). We have been in single figures now for seventy-two days.
We should remind ourselves that when prevalence is low, as it was last summer, and as it is now, the small false positive rate of the PCR test becomes important, having a pronounced distorting effect on new cases data. To gain a better understanding of this, readers could do worse than start with our 'Covid Kidology' articles published in the autumn of 2020. If you don't have time or inclination to read those articles, here, without doing too much research, is the essence of it: In the UK we currently do about 300,000 PCR tests per day and at the time of writing (Thursday, 27 May) the tests came up with about 3500 positive results. The last time I checked, the best estimate we have of the false-positive rate is between 0.4 and 4.3%. (ONS, October 2020). Let's call it 1% which would mean about 3000 out of the 3500 positive results would be false. If the false-positive rate was 1.17%, they would all be false. In either (or any) case, we could never be free of the disease - unless, of course, we stopped testing! Think about it. The latest (Delta) variant is now overwhelmingly the dominant strain. PHE reports that almost all new cases are the Indian variant which is much more contagious than the Kent flavour, with different symptoms (more like a heavy cold). The question is; are the vaccines fending it off? Early signs are good but 'freedom day' has been put back 'til 19 July 2021.
The chart below shows cases by specimen date, so the number includes cases added to earlier days in a process of continuous revision. The chart was updated on 17/6/2021.
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