Thanks for asking about this. It's a rather subtle point, and I'm not sure I can explain it clearly. The fatality rate (alpha) in the model is not the true infection fatality rate. What it does is divide the population of infected individuals into two categories, one of which recovers at a rate of 1/D per day and the other dies at a rate of 1/H per day. (D is the contagious period and H is the treatment period. ) If D < H, patients recover more quickly than they die, and you get fewer fatalities. If D = H, the result should be what you had expected (although the model seems slightly off, and I'm looking to see why.) Another issue with the terminology in SIRD and SEIRD models is that "contagious period" is not exactly the right term. Individuals being treated remain in the infectious compartment and can still spread the disease. In principle, It might be more precise to introduce another (treatment) compartment, where the reproduction number could be adjusted independently.
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