Mortality rate (more accurately case fatality rate, but it’s quicker to type mortality rate) could only be lower if prevalence is actually higher than reported.
Mortality rate is deaths due to disease divided by disease prevalence. When the Wuhan coronavirus was first discovered it was in a relatively isolated population. Deaths and prevalence were simpler to calculate. As the virus has spread, deaths remained easy to calculate, but prevalence became much more difficult. That’s why it appears as if the mortality rate has increased. This phenomenon was seen with SARS where it looked as if there was increasing virulence until better screening and testing resulted in more accurate prevalence numbers.
Right now this remains a somewhat novel coronavirus with a higher virulence than your run of the mill coronavirus but nowhere near SARS or MERS. Until widespread screening or mandatory testing take place, prevalence will be underestimated leading to falsely high mortality rates. Except for one outlier that I know of (healthy 36 year old male), virtually all of the death have been in the elderly with significant pre-existing conditions. This is also typical of a relatively low virulence strain.
