Covid's natural born victims

Why do some people fall seriously ill and die with Covid whilst others are asymptomatic? A remarkable characteristic of Covid-19 is the way it affects individuals so differently: Around 40% of people infected with SARS-CoV-2 are asymptomatic. A further 40% experience mild, upper respiratory tract symptoms. The remaining 20% develop pneumonia, of which 10% will become hypoxic, leading to critical illness in around 3%. In addition to age, sex and underlying health issues, the ability to deploy key immunomodulatory cytokines has emerged as an important risk factor for mortality.