COVID-19 Infection Survey pilot and the reporting of new cases

You asked

I have looked at your recent Coronavirus (COVID-19) Infection Survey pilot reports regarding new cases dated 31st July 2020.

Please provide scientific evidence that categorically proves that the individuals tested did not have coronavirus infection prior to the date tested, i.e. that they were not infected with coronavirus in period 1st October 2019 to 31st July of 2020.

https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/conditionsanddiseases/bulletins/coronaviruscovid19infectionsurveypilot/31july2020

We said

Thank you for your request relating to the Coronavirus (COVID-19) Infection Survey pilot.

In the course of our study, we ask every participant aged 2 years or older in each household to receive a nose and throat swab, whether or not they had reported symptoms. Every participant is swabbed once. Participants are also invited to have repeat tests every week for the first five weeks and monthly tests for a period of 12 months in total.

We estimate the number of new infections (the incidence rate) by using all swab test results in a Bayesian model to estimate the incidence rate of new infections for each different type of respondent (by age, sex and region) who tested negative when they first joined the study.

A new infection must follow at least one negative nose and throat swab result to be included in our model. This means that participants whose first swab test in the study was positive are excluded from our incidence estimates, as they could reasonably have been positive for COVID-19 prior to the start of our study.

As at least two repeated swab test visits are required to be included in the incidence analysis and we started recruiting participants on 26 April 2020, only data from 11 May onwards are included in the incidence rate model.

This means that, during the testing process, individuals included in our estimates of new infections did not have COVID-19 when they had previously provided a swab. The period of time in which they were tested could have started at any point from the 26 April up until the most reporting week (20 August).

You can find more detailed information on the methods used to calculate estimates of new infections in our methodology article.