The author spends a section on that, including a chart. Search for this passsage to see the chart:
The IFR (Infected Fatality Rate4) for the original variant was about 0.01% for those in their 20s and 0.2% for those in their 40s.
Direct link to the chart https://cdn.substack.com/image/fetch/f_auto,q_auto:good,fl_p...