Oliver Johnson

Covid cases are falling – but don’t think it’s all over

The official Covid-19 dashboard has turned green and cases appear to have peaked, at least for now. You hear a lot of people say ‘cases don’t matter’, but they are a leading indicator of hospitalisations and deaths — and so a peak in cases (if confirmed by the ONS survey) should lead to a peak in hospitalisations in future weeks.


So, is that it? Are we done? It would be unwise to estimate the R numbers for a few days. Not because I’m disappointed that numbers are falling, but because you can’t do linear regression on non-linear data. And there is still a lot of uncertainty.

However, for the first time in 18 months, there’s been a fall in cases that can’t be easily explained by a national lockdown. That’s great news.

However, where it gets murkier is when you try to work out why this fall is happening, and try to figure out what it means. Remember: cases aren’t infections, they simply represent the fraction of infected people who happened to test positive. Cases are down, but so are tests, and anecdotally there are reports of the test system slowing down. That’s not enough to explain this (cases are currently decreasing faster than tests — see below graph), but of course the number of tests is a factor.

And we need to bear in mind the ONS weekly infection survey, based on a random sampling of the population, which also picks up asymptomatic cases. The ONS uses this to produce modelled estimates of the number of people who would test positive — this data should rise or fall irrespective of how many tests were being offered or how many were coming forward for a test. The below shows data to 17 July and there isn’t any sign of a fall.

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