But until treatments/vaccines were developed, everyone on Earth had the disease-equivalent, sometimes-fatal status of "no prior immunity to COVID".
Sure, the scale is massively different for a young healthy person: "susceptible to COVID" would mean a moderate chance of catching the disease (growing over time), and then a small chance of death or previous complications. But the "susceptibility" is also a "health condition" with nonzero risk-of-death.
And for society as a whole, COVID passed cancer as a cause of death last year, and in January 2021 has been killing about twice as many people as all cancers combined.
So "diagnosed-with-cancer" versus "susceptible-to-COVID" shouldn't be that different in terms of the kinds of risks we'd let informed people take to address that danger.
What's being proposed here is for people to sign up and then inject a potentially deadly disease into them.