Ascertainment rate of SARS-CoV-2 infections from healthcare and community testing in the UK

https://doi.org/10.1016/j.jtbi.2022.111333Get rights and content
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Highlights

  • Analysis of reported SARS-CoV case numbers in the UK and a national infection survey.

  • We introduce methods to estimate case ascertainment and daily infection numbers.

  • Around 30% of infections were ascertained; this varies depending on age and variant.

  • Changes over time reflect changes in attitudes and test availability.

  • Infection fatality rate peaked in early 2021 and declined as vaccination increased.

Abstract

The proportion of SARS-CoV-2 infections ascertained through healthcare and community testing is generally unknown and expected to vary depending on natural factors and changes in test-seeking behaviour. Here we use population surveillance data and reported daily case numbers in the United Kingdom to estimate the rate of case ascertainment. We mathematically describe the relationship between the ascertainment rate, the daily number of reported cases, population prevalence, and the sensitivity of PCR and Lateral Flow tests as a function time since exposure. Applying this model to the data, we estimate that 20%–40% of SARS-CoV-2 infections in the UK were ascertained with a positive test with results varying by time and region. Cases of the Alpha variant were ascertained at a higher rate than the wild type variants circulating in the early pandemic, and higher again for the Delta variant and Omicron BA.1 sub-lineage, but lower for the BA.2 sub-lineage. Case ascertainment was higher in adults than in children. We further estimate the daily number of infections and compare this to mortality data to estimate that the infection fatality rate increased by a factor of 3 during the period dominated by the Alpha variant, and declined in line with the distribution of vaccines. This manuscript was submitted as part of a theme issue on “Modelling COVID-19 and Preparedness for Future Pandemics”.

Keywords

SARS-CoV-2
Ascertainment rate
Infection fatality rate

Data availability

Data and code are available at https://github.com/EwanColman/Estimating_SARS-CoV-2_case_ascertainment.

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