  Several SARS-CoV-2 variants that evolved during the COVID-19 pandemic have
appeared to differ in severity, based on analyses of single-country datasets.
With decreased SARS-CoV-2 testing and sequencing, international collaborative
studies will become increasingly important for timely assessment of the
severity of newly emerged variants. The Joint WHO Regional Office for Europe
and ECDC Infection Severity Working Group was formed to produce and pilot a
standardised study protocol to estimate relative variant case-severity in
settings with individual-level SARS-CoV-2 testing and COVID-19 outcome data
during periods when two variants were co-circulating. To assess feasibility,
the study protocol and its associated statistical analysis code was applied by
local investigators in Denmark, England, Luxembourg, Norway, Portugal and
Scotland to assess the case-severity of Omicron BA.1 relative to Delta cases.
After pooling estimates using meta-analysis methods (random effects estimates),
the risk of hospital admission (adjusted hazard ratio [aHR]=0.41, 95% CI
0.31-0.54), ICU admission (aHR=0.12, 95% CI 0.05-0.27), and death (aHR=0.31,
95% CI 0.28-0.35) was lower for Omicron BA.1 compared to Delta cases. The aHRs
varied by age group and vaccination status. In conclusion, this study has
demonstrated the feasibility of conducting variant severity analyses in a
multinational collaborative framework. The results add further evidence for the
reduced severity of the Omicron BA.1 variant.
