You've linked a document created during the 2009 pandemic, when CFR rates are likely to be too high.
https://www.bmj.com/content/339/bmj.b2840
> At first sight, the data seem to imply that this new virus is relatively mild, with case fatality ratios around 0.5%, similar to the upper range of that seen for seasonal influenza2 and relatively low hospitalisation ratios. However, the case fatality ratio seems to vary substantially between countries, and deaths have occurred in much younger people than is the case for seasonal influenza.3 4
> There are many reasons why simple interpretations of these crude figures at the beginning of a pandemic may be misleading both in terms of assessing severity and in making comparisons between countries. Here, we discuss some of the important mechanisms resulting in biases, propose study designs and associated statistical methods to estimate the case fatality ratio given these limitations, and show their strengths using simulated data. The two main sources of bias in estimates of the case fatality ratio we consider stem from shifts in case ascertainment (over time, efforts may become more focused on the most severe cases, leading to an overestimation of the case fatality ratio) and from the inevitable delay between symptom onset and death, which in the early phase of the epidemic can lead to underestimation of the case fatality ratio if it is not adjusted for.
> A natural definition for the case fatality ratio is the ratio of the total number of deaths from a disease divided by the total number of cases. In a fully ascertained (and complete) epidemic, this simple method works perfectly. However, in most infectious diseases there is underascertainment of cases as people who are asymptomatic or have mild infection will be less likely to present to health care, and if they do present they will be less likely to be tested and confirmed. It is therefore likely that there will be a bias towards diagnosis of more severe cases (fig 1⇓), with the result that the case fatality ratio and other measures of severity are overestimated. Furthermore, this underascertainment will change as an epidemic matures. Initially increased awareness by patients and doctors may lead to high ascertainment, but as cases increase and systems are overwhelmed, only a proportion will be tested (potentially those with links to other confirmed cases), making it difficult to understand the scale of under-reporting.