Hospital admission and emergency care attendance risk for SARS-CoV-2 delta (B.1.617.2) compared with alpha (B.1.1.7) variants of concern: a cohort study

Background: The SARS-CoV-2 Delta (B.1.617.2) variant was first detected in England in March 2021. It has since rapidly become the predominant lineage, owing to high transmissibility. It is suspected that the Delta variant is associated with more severe disease than the previously dominant Alpha (B.1.1.7) variant. Methods: Individual-level data on 43,338 COVID-19-positive cases (median age 31 years, inter-quartile range 17-43) in England between 29 March and 23 May 2021 and whose positive specimen had undergone whole genome sequencing were linked to routine healthcare datasets on vaccination, emergency care attendance, hospital admission and mortality. Hospital attendance and admission outcomes were compared between cases with sequencing-confirmed Delta and Alpha variants. Stratified Cox regression was used to adjust for age, sex, ethnicity, deprivation, recent international travel, area of residence, calendar period and vaccination status. Findings: There were 960 hospital admissions (2.5%) and 1,946 hospital admissions or emergency care attendances (4.5%) within 14 days. The adjusted hazard ratio (HR) for Delta vs Alpha cases was 2.26 (95% CI 1.32-3.89) of hospital admission within 14 days, and 1.45 (95% CI 1.08-1.95) of emergency care attendance or hospital admission within 14 days. Most cases were unvaccinated (32,078/43,338, 74%). The HRs for vaccinated Delta vs Alpha cases were similar to the HRs for unvaccinated Delta vs Alpha cases, but the precision for the vaccinated subgroup was low. Interpretation: This large national study found a higher emergency care attendance or hospital admission risk for COVID-19 cases infected with the Delta compared with the Alpha variant. While rates of hospital care among vaccinated individuals were low overall, results suggest that outbreaks of the Delta variant in unvaccinated populations may lead to a greater burden on healthcare services than the Alpha variant. Funding: Medical Research Council and MRC UKRI/DHSC NIHR.