Counting reinfections

People testing positive often count once, so what about reinfections?

Anthony B. Masters

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The ITV political editor Robert Peston claimed on Twitter:

In case you didn’t know — and I didn’t till a senior government official told me — the daily tally of infections seriously understates the actual number of infections, because if you are sick with [Covid-19] today but had it any time in the past (even last spring) your new bout is not included in the daily dashboard figures. We know people are being reinfected.

The basics of the case

Many public health agencies publish daily epidemic statistics, including new SARS-CoV-2 cases. If you test the same person many times, one infection could produce several positive test results. To counter duplication under the same infection, agencies and health departments often count only one positive from each person.

Even within the United Kingdom, there are small differences in case definitions. All cases need confirmation from polymerase chain reaction tests in Northern Ireland, Scotland, and Wales. Public Health England counts lateral flow positive results — only excluding those with a negative PCR test within three days.

That de-duplication is stated several times on the PHE dashboard:

People tested positive more than once are only counted once, on the date of their first positive test.

The Northern Ireland Department of Health counts the most recent positive.

The problem of only counting people once is the exclusion of genuine reinfections. How would adding reinfections affect headline numbers?

Up to 4th July 2021, there were 4.3m first-positive test results in England. Public Health England defines a ‘possible reinfection’ as two positive results at least 90 days apart, with around 23,000 such cases. That is about 0.5% of the confirmed case total. In the Office for National Statistics infection survey, there were 15 (13 to 18) reinfections per 100,000 participant-days at risk.

A ‘confirmed reinfection’ needs sequencing of both specimens, to check they are…

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Anthony B. Masters

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.