Estimating disease severity of Omicron and Delta SARS-CoV-2 infections

The Omicron variant of SARS-CoV-2 has been reported to cause milder disease in adults but lead to increased hospital admissions in children. How can we compare disease severity in Omicron and Delta infections, and how should differences be interpreted?

Measuring COVID-19 disease severity in a population has been important for understanding the public health impact of each variant of concern. It also impacts immunologists and virologists closely as it reflects population immunity and the mechanisms of viral infection. A watershed moment in the COVID-19 pandemic was the emergence of the Omicron (B.1.1.529) variant of SARS-CoV-2 with widespread reports of lower disease severity relative to previous variants such as Delta (B.1.617.2). The lower disease severity seen in populations during the Omicron wave of the SARS-CoV-2 pandemic infection1 can be attributed to changes in the virus that limit its ability to spread in the lungs and, probably most importantly, to increased immunity in the population from previous SARS-CoV-2 infection and vaccination2. However, in children, Omicron infections led to more hospital admissions than in previous waves3,4. Does this mean that Omicron is causing more severe disease in children, or is the difference based on how disease severity is defined?..