This supplementary appendix provides additional details and analyses for the study comparing the risks of hospitalisation and death associated with SARS-CoV-2 Omicron (B.1.1.529) and Delta (B.1.617.2) variants in England. The study used a cohort of COVID-19 cases identified between 29 November 2021 and 9 January 2022. The data selection process is detailed in Figure S1, which shows the flow of cases through the study. Figure S2 presents the positive and negative predictive values for using S-gene target failure (SGTF) to classify Omicron or Delta cases. The predictive values for Omicron increased from 78% at the start of the study period to over 99% after 8 December, while the NPV for Delta decreased due to the BA.2 variant. Figure S3 shows the descriptive frequencies of all cases, including those not meeting the study criteria. Figure S4 presents the distribution of cases and hospitalisations in the 0-9 year age band by variant. Tables S1 to S12 provide detailed statistical analyses, including hazard ratios (HRs) for hospitalisation and mortality, adjusted for various factors such as vaccination status, prior infection, and age groups. Sensitivity analyses are conducted to assess the robustness of the findings, considering different definitions of hospitalisation, epidemic phase bias, and under-ascertainment of past infections. The supplementary methods describe the sensitivity analysis adjusting for under-ascertainment of past infections, using imputation methods based on population infection prevalence data. The analysis accounts for the possibility that some individuals without known past infections may have had undetected infections. The results show that adjusting for under-ascertainment does not significantly affect the HR estimates. The study highlights the importance of considering various factors in the analysis of SARS-CoV-2 variants and their impact on hospitalisation and mortality risks.This supplementary appendix provides additional details and analyses for the study comparing the risks of hospitalisation and death associated with SARS-CoV-2 Omicron (B.1.1.529) and Delta (B.1.617.2) variants in England. The study used a cohort of COVID-19 cases identified between 29 November 2021 and 9 January 2022. The data selection process is detailed in Figure S1, which shows the flow of cases through the study. Figure S2 presents the positive and negative predictive values for using S-gene target failure (SGTF) to classify Omicron or Delta cases. The predictive values for Omicron increased from 78% at the start of the study period to over 99% after 8 December, while the NPV for Delta decreased due to the BA.2 variant. Figure S3 shows the descriptive frequencies of all cases, including those not meeting the study criteria. Figure S4 presents the distribution of cases and hospitalisations in the 0-9 year age band by variant. Tables S1 to S12 provide detailed statistical analyses, including hazard ratios (HRs) for hospitalisation and mortality, adjusted for various factors such as vaccination status, prior infection, and age groups. Sensitivity analyses are conducted to assess the robustness of the findings, considering different definitions of hospitalisation, epidemic phase bias, and under-ascertainment of past infections. The supplementary methods describe the sensitivity analysis adjusting for under-ascertainment of past infections, using imputation methods based on population infection prevalence data. The analysis accounts for the possibility that some individuals without known past infections may have had undetected infections. The results show that adjusting for under-ascertainment does not significantly affect the HR estimates. The study highlights the importance of considering various factors in the analysis of SARS-CoV-2 variants and their impact on hospitalisation and mortality risks.