Risk of covid-19 related deaths for SARS-CoV-2 omicron (B.1.1.529) compared with delta (B.1.617.2): retrospective cohort study

Abstract

Objective: To assess the risk of covid-19 death after infection with omicron BA.1 compared with delta (B.1.617.2).

Design: Retrospective cohort study.

Setting: England, United Kingdom, from 1 December 2021 to 30 December 2021.

Participants: 1 035 149 people aged 18-100 years who tested positive for SARS-CoV-2 under the national surveillance programme and had an infection identified as omicron BA.1 or delta compatible.

Main outcome measures: The main outcome measure was covid-19 death as identified from death certification records. The exposure of interest was the SARS-CoV-2 variant identified from NHS Test and Trace PCR positive tests taken in the community (pillar 2) and analysed by Lighthouse laboratories. Cause specific Cox proportional hazard regression models (censoring non-covid-19 deaths) were adjusted for sex, age, vaccination status, previous infection, calendar time, ethnicity, index of multiple deprivation rank, household deprivation, university degree, keyworker status, country of birth, main language, region, disability, and comorbidities. Interactions between variant and sex, age, vaccination status, and comorbidities were also investigated.

Results: The risk of covid-19 death was 66% lower (95% confidence interval 54% to 75%) for omicron BA.1 compared with delta after adjusting for a wide range of potential confounders. The reduction in the risk of covid-19 death for omicron compared with delta was more pronounced in people aged 18-59 years (number of deaths: delta=46, omicron=11; hazard ratio 0.14, 95% confidence interval 0.07 to 0.27) than in those aged ≥70 years (number of deaths: delta=113, omicron=135; hazard ratio 0.44, 95% confidence interval 0.32 to 0.61, P<0.0001). No evidence of a difference in risk was found between variant and number of comorbidities.

Conclusions: The results support earlier studies showing a reduction in severity of infection with omicron BA.1 compared with delta in terms of hospital admission. This study extends the research to also show a reduction in the risk of covid-19 death for the omicron variant compared with the delta variant.

Fig 1

Hazard ratio for covid-19 death for omicron BA.1 infection compared with delta infection using fully adjusted (model 4) and alternative models. Black line shows the null (omicron BA.1 no different to delta). Model 1 adjusted for sex, age (natural spline), vaccination status and previous infection; model 2 also adjusted for calendar time (natural spline); model 3 also adjusted for ethnicity, index of multiple deprivation rank (natural spline), household deprivation, university degree, keyworker status, country of birth, main language, region and disability; model 4 also adjusted for comorbidities

Fig 2

Hazard ratio for covid-19 death for omicron BA.1 infection versus delta infection by sex, age, and vaccination status. To investigate the interaction between variant type and sex, the model was fully adjusted (model 4, adjusted for age, sex, vaccination status, previous infection, calendar time, socioeconomic factors, and comorbidities) with an interaction term for variant and sex. For the interaction between variant and age, the fully adjusted model also included a variable for age group (18-59, 60-69, or ≥70). For the interaction between variant and vaccination status, additional interaction terms were included between variant and vaccination categories and adjusted for an interaction between variant and age