Epidemiological Parameters of SARS-CoV-2 in the UK during the 2023/2024 Winter: A Cohort Study

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 into multiple epidemiological parameters: namely, the duration of positivity, test sensitivity, and the incubation period. 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 8.55 days (95% CrI: 7.65 days, 9.44 days) in the youngest age group compared to 10.27 days (95% CrI: 9.85 days, 10.71 days) in the oldest age group. Similarly, test sensitivity, as a function of time since symptom onset, decays fastest in the youngest age group, reaching a minimum sensitivity of 0.26 (95% CrI: 0.16, 0.37) compared to 0.54 (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. Combining the duration of positivity and test sensitivity, we estimate the probability of returning a positive test. Close to symptom onset date, this probability is approximately 95%. However, this rapidly drops off, dropping below 5% after 11.3 days (95% CrI: 9.7 days, 13 days) for the youngest age group and 16.2 days (95% CrI: 15.4 days, 17.1 days) for the oldest age group. For the incubation period, there is no clear pattern by age. Across all age groups, the mean incubation period is 2.52 days (95% CrI: 2.42 days, 2.62 days). This is shorter than the most recent estimates for Omicron BA.5, which is in line with earlier research that found replacing variants had shorter incubation periods.