That's not a valid assumption.
There a dozens of reasons to be immune compromised - while older people do have weaker immune systems as a general rule....anyone in cancer treatment does, almost any auto-immune disease, one associate has chrons for instance and takes Remicade, my kid has type one diabetes AND being treated for brain cancer.
They are all immune compromised until they die and even if they survive as I did, it takes a while to get back to healthy.
this is not over 700,000 cases age 20-59 ....
COVID-19 in Ontario: January 15, 2020 to March 5, 2022
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and of course many treatments have been delayed due to covid putting added pressure on overloaded hospitals so keeping a mask on in grocery stores and other venues is not such a big ask.