This study analyzes a dataset linking 2,245,263 positive SARS-CoV-2 community tests and 17,452 deaths associated with COVID-19 in England from November 1, 2020, to February 14, 2021. The analysis focuses on the B.1.1.7 variant, which was first detected in the UK in September 2020 and is known for its higher transmissibility. By identifying the presence or absence of mutations that prevent PCR amplification of the spike gene (SGTF), the study estimates that the hazard of death associated with SGTF is 55% (95% CI, 39–72%) higher than in cases without SGTF, after adjusting for age, sex, ethnicity, deprivation, residence in a care home, local authority, and test date. This corresponds to an absolute risk increase of 0.2% for a 55–69-year-old man, from 0.6% to 0.9% within 28 days of a positive test. Correcting for misclassification and missing data, the hazard ratio increases to 61% (42–82%). The findings suggest that the B.1.1.7 variant not only increases transmissibility but also causes more severe illness. The study also examines the characteristics of the study population, the prevalence of SGTF over time, and the robustness of the results through various sensitivity analyses.This study analyzes a dataset linking 2,245,263 positive SARS-CoV-2 community tests and 17,452 deaths associated with COVID-19 in England from November 1, 2020, to February 14, 2021. The analysis focuses on the B.1.1.7 variant, which was first detected in the UK in September 2020 and is known for its higher transmissibility. By identifying the presence or absence of mutations that prevent PCR amplification of the spike gene (SGTF), the study estimates that the hazard of death associated with SGTF is 55% (95% CI, 39–72%) higher than in cases without SGTF, after adjusting for age, sex, ethnicity, deprivation, residence in a care home, local authority, and test date. This corresponds to an absolute risk increase of 0.2% for a 55–69-year-old man, from 0.6% to 0.9% within 28 days of a positive test. Correcting for misclassification and missing data, the hazard ratio increases to 61% (42–82%). The findings suggest that the B.1.1.7 variant not only increases transmissibility but also causes more severe illness. The study also examines the characteristics of the study population, the prevalence of SGTF over time, and the robustness of the results through various sensitivity analyses.