SARS-CoV-2 test sensitivity and duration of positivity in the UK during the 2023/2024 Winter: A prospective cohort...

Estimating epidemiological parameters is essential for informing an effective public health response during waves of infectious disease transmission. However, many parameters are challenging to estimate from real-world data and rely on human challenge studies or mass community testing. During Winter 2023/2024, a community cohort study of SARS-CoV-2 was conducted across households in England and Scotland. From this survey, questionnaire data and follow-up testing protocols provided valuable data on the duration of positivity and test sensitivity for lateral flow device (LFD) tests. Here, Bayesian statistical modelling methods are developed and applied to estimate the underlying parameters. The duration of LFD positivity is found to increase with increasing age, with a mean of 9.1 days (95% CrI: 8.4 days, 9.9 days) in the 18 to 34 years age group compared to 10.8 days (95% CrI: 10.3 days, 11.3 days) in the 75 years and over age group. Sex is found to have no impact on the duration of positivity. LFD test sensitivity at the time of symptom onset is very high, with an estimated sensitivity of 95% (95% CrI: 92%, 98%) across all age groups. As a function of time since symptom onset, LFD test sensitivity decays fastest in the youngest age group, reaching a minimum sensitivity of 0.26 (95% CrI: 0.16, 0.37) compared to 0.53 (95% CrI: 0.46, 0.6). Such patterns are expected since younger individuals experience less severe symptoms of COVID-19 and are likely to clear the virus faster. Females are found to have a slightly faster rate at which sensitivity decreases, but the same minimum sensitivity as Males. Combining the duration of positivity and test sensitivity distributions, we estimate the probability of returning a positive LFD test. Close to the symptom onset date, this probability is approximately 95%. However, this rapidly drops off, dropping below 5% after 13.8 days (95% CrI: 11.0 days, 17.3 days) for the youngest age group (3 to 17 years) and 17.8 days (95% CrI: 16.6 days, 19.2 days) for the 75 years and over age group. Although the probability of returning a positive LFD test rapidly drops off, it remains very high close to the time of symptom onset, which is when individuals are expected to be the most infectious.