Counting reinfections

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

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.