Case fatality/mortality rate is dependent on a number of variables, including quality and availability of healthcare, the efficacy and availability of treatments, and the pre-existing health of a population. In turn, the availability of healthcare is related to the degree of success in controlling disease transmission, which itself is dependent on a number of response factors. The higher the quality of healthcare, the more successfully transmission is limited, the better the available treatments, and the healthier the population at risk, the lower the case mortality rate, and the converse is true, too. I suggest you read Wuhan and Hubei COVID-19 mortality analysis reveals the critical role of timely supply of medical resources at https://www.journalofinfection.com/article/S0163-4453(20)30145-6/pdf , and particularly the charts on page six that show daily mortality rate, patient-healthcare worker ratio, availability of critical care and other hospital beds, and the recovery rate. The reported mortality rates varied by time, location and healthcare system capacity, from as high as 3.5% in Wuhan City, but overtime the rates for all regions dropped to flu-like levels. The findings of this paper are that the mortality rate was inversely related to the number of healthcare workers and beds. This implies that mortality rates may be much higher in some communities than others. It could be that countries with very limited healthcare systems, little ability to limitation infection transmission, limited access to treatments, and unhealthy populations could experience mortality rates much higher than even Wuhan City, much less Germany.
Taking data from a single advanced country, such as Germany, and using the fatality rate there to estimate the fatality rate in other countries is not reasonable, unless the other countries have a similar healthcare system, similarly available treatments, similar transmission limitation success, and similarly healthy population. Reported data varies in quality, and it is likely that some developing countries will not have the capability to collect and report cases or fatalities, and even some developed countries might do this poorly, too.
Ultimately, may be possible – after the fact – to estimate a reasonably accurate global fatality rate based on random serum testing and statistical estimates of additional deaths , but so what? That may be of historical interest, but arguing about it now really isn’t useful. Moreover, if one collects the mortality rates of HCID listed diseases, it becomes apparent that almost all of these rates are estimates that vary particularly by the characteristics of the communities impacted and the availability of effective treatmetns.
For COVID-19, at this time the mortality rate from a range of countries with substantially different characteristics is not known – most of the data to date comes from advanced economies. Some countries may experience low mortality rates, like Germany. Some very populous poorer countries may experience much higher mortality rates. What matters now is collecting evidence from countries with early experience, understanding the variables that impact mortality rate, and taking actions to lower the mortality rate. We know COVID-19 has the capacity to overwhelm even advanced healthcare systems where infection transmission is not sufficiently suppressed, and that mortality rates will be higher to the degree that healthcare systems fail. We know that the mortality rate can be much higher than the flu in such circumstances and that a significant number of victims require advanced life support or they will die. Therefore, COVID1-19 creates a substantial risk of death, much greater than the seasonal flu. Hypothesizing that the ultimate mortality rate is this or that is an unhelpful distraction.
On the other hand, estimating a range of mortality rates based on community characteristics, case severity rates, and herd immunity in different communities is helpful to understanding a possible range of impacts and the tactics that can best limit the impacts of COVID-19 and the potential duration of future impacts.
