Epidemiological drivers of transmissibility and severity of SARS-CoV-2 in England

The COVID-19 pandemic has been characterised by periods of dominance of different SARS-CoV-2 variants. In this mathematical modelling study, the authors investigate the epidemiological properties of successive variants in England until early 2022 and quantify the impacts of control measures. As the SARS-CoV-2 pandemic progressed, distinct variants emerged and dominated in England. These variants, Wildtype, Alpha, Delta, and Omicron were characterized by variations in transmissibility and severity. We used a robust mathematical model and Bayesian inference framework to analyse epidemiological surveillance data from England. We quantified the impact of non-pharmaceutical interventions (NPIs), therapeutics, and vaccination on virus transmission and severity. Each successive variant had a higher intrinsic transmissibility. Omicron (BA.1) had the highest basic reproduction number at 8.3 (95% credible interval (CrI) 7.7-8.8). Varying levels of NPIs were crucial in controlling virus transmission until population immunity accumulated. Immune escape properties of Omicron decreased effective levels of immunity in the population by a third. Furthermore, in contrast to previous studies, we found Alpha had the highest basic infection fatality ratio (2.9%, 95% CrI 2.7-3.2), followed by Delta (2.2%, 95% CrI 2.0-2.4), Wildtype (1.2%, 95% CrI 1.1-1.2), and Omicron (0.7%, 95% CrI 0.6-0.8). Our findings highlight the importance of continued surveillance. Long-term strategies for monitoring and maintaining effective immunity against SARS-CoV-2 are critical to inform the role of NPIs to effectively manage future variants with potentially higher intrinsic transmissibility and severe outcomes.

Automated summary

In this study, the epidemiological properties of different SARS-CoV-2 variants in England until early 2022 were examined using mathematical modeling. The impact of control measures, including non-pharmaceutical interventions, therapeutics, and vaccination, on virus transmission and severity was quantified. Each successive variant had a higher transmissibility, with Omicron being the most transmissible. NPIs played a crucial role in controlling virus transmission. Immune escape properties of Omicron reduced population immunity. Alpha had the highest infection fatality ratio, followed by Delta, Wildtype, and Omicron. Continued surveillance and long-term strategies for maintaining effective immunity are important to manage future variants.