Posted on 9 March 2020
Varying reports of the mortality rate for the novel coronavirus have caused some confusion. This is because calculating the true mortality rate is not as simple as it may at first seem. A large proportion of those who become infected will not have severe symptoms, and hence will go unreported. Not accounting for this leads to overestimation of mortality rate. Conversely, if calculations include cases that have not yet run their course, the result will be an underestimate of mortality, as some of those cases will later result in death.
A preprint study used statistical methods to adjust for these factors, demonstrating how they affect estimates for different age groups. The figures show the importance of age: when predicted asymptomatic infections are accounted for, the adjusted mortality rate is approximately 1 in 22 for those in their 60s, 1 in 10 for those in their 70s, and 1 in 6 for those who are 80 or older. For symptomatic cases only, those numbers are roughly twice as high.
Age in and of itself is not the only factor at play here, however. Elderly people are significantly more likely to have hypertension, heart disease and respiratory disease, all of which increase the risk of death from coronavirus infection. These morbidities interact with age to determine overall risk, and so for any age group, the mortality rate for someone with none of these conditions will likely be lower than the figures presented here.
Adjusted age-specific case fatality ratio during the COVID-19 epidemic in Hubei, China, January and February 2020: https://doi.org/10.1101/2020.03.04.20031104