A question central to the Covid-19 pandemic is why the Covid-19 mortality rate varies so greatly across countries, from over 16% in France and Belgium to less than 0.1% in Singapore and Qatar. Such wide variation implies that there are factors other than patient characteristics that determine Covid-19 mortality, such as government response. Patient-level studies have shown that Covid-19 mortality can be explained by age, obesity, and underlying diseases, such as hypertension, diabetes, and coronary heart disease, etc.. as well as clinical symptoms, complications, hospital care, previous immunity and virus mutations. These findings help health professionals to identify high-risk patients. However, this evidence alone may not be sufficient to support effective policies for reducing Covid 19 mortality.