The Incidence and Prevalence of SARS-CoV-2 in the UK Population from the UKHSA Winter COVID Infection Study

The SARS-CoV-2 virus continues to cause substantial morbidity and mortality, particularly during the winter period. The Winter Covid Infection Study (WCIS) ran from the 14 ^th of November 2023 to the 7 ^th of March 2024, and enabled the UK Health Security Agency to publish fortnightly estimates of the incidence and prevalence of SARS-CoV-2. Testing was performed using Lateral Flow Devices (LFD), and a repeat testing design was used to estimate key epidemiological parameters of SARS-CoV-2. This facilitated the estimation of time-varying prevalence, incidence, and test sensitivity. A Bayesian multilevel regression and poststratification model was developed to produce representative and unbiased estimates. In England and Scotland, prevalence peaked at 4.54% (95% CI: 3.90 to 5.24), and incidence peaked at 498 (95% CrI: 429 to 585) new infections per 100,000 individuals per day. The average LFD test sensitivity in England and Scotland during the study was estimated to be 72.1% (95% CrI: 70.3, 74.0), though due to epidemic phase bias this varied from a minimum value of 68.6% (95% CrI: 66.4 to 70.7) to a maximum value of 77.2% (95% CrI: 75.3 to 79.2). The novel study design of WCIS addressed key survey design challenges faced by previous large-scale SARS-CoV-2 population prevalence studies. The study demonstrated the utility and cost-benefit of LFD tests in large community surveys of prevalence.