Why we should use, and be optimistic about, the ONS COVID-19 infections data

COVID-19 infections have been steadily declining in the UK over the past few months, thanks to the national lockdown and the vaccination drive.

The Office for National Statistics (ONS) publish weekly statistics on their ‘COVID-19 Infection Survey’ (here). This is used to calculate the number of new COVID-19 cases by randomly testing 1000s of individuals in the UK regardless of whether they have symptoms. This therefore paints a more accurate picture than the official data on positive tests, as it includes individuals that are asymptomatic. This is illustrated in the below chart:

Source: UK Government, ONS

Therefore, we should pay more attention to the ONS survey, as it includes asymptomatic individuals. In addition, the official cases data is affected by the number of tests carried out (more tests = more positive results = more official cases), whereas the ONS survey controls for the number of random tests that it carries out.

We can thus be optimistic when the ONS infection survey shows a decline in the number of COVID-19 infections. The below charts illustrate the results in England from the ONS infection survey. The survey tests a random sample of the population and then extrapolates the results to give an estimate for all of England. Since this is an estimate, there remains some uncertainty. The shaded areas in the below charts represent what is known as a ‘confidence interval’. This means that we can 95% confident that the estimated values fall within the shaded areas of the below charts.

As the above charts show, infections levels are drastically lower than the peak seen around the turn of the year; this is clearly a cause for optimism. What is perhaps even more encouraging is that new infections are still following a downwards trajectory, despite the reopening of schools and the recent increases in social contact. It will take a few weeks before the effects of retail and hospitality reopening are seen and so the ONS data should be carefully watched over the coming weeks. Nevertheless, the available data offers confidence that we remain on track to ease COVID-19 restrictions in the UK, with infections now back to the level last seen at the end of summer 2020.

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