Reporting COVID-19 Cases

Bulletin 95 | Chris Daykin

In order to draw meaningful conclusions about the rate of new infections from COVID-19, the numbers of positive test results reported each day should be expressed as a percentage of the number of tests carried out that day, in order to display the positivity rate. The number of positive tests on its own can be distorted by several factors.

Comparison with numbers of positive results at the beginning of the pandemic illustrates this, since on the graphs shown in the daily government briefings the hump of new infections in April looks tiny relative to the hump in the second wave, whereas actually the rate of positivity in early April rose to about 27%, with a weekly average peaking at 23% on 8 April. Availability of testing was much more limited then, however, which will have increased the positivity rate observed.

In the second hump the rate of positivity hit a maximum of 15.4% on 29 December. The weekly average peaked at 13.0% on 4 January and has been falling quite steadily ever since then to a figure of 7.5% today.