Belief is not a measurement. Hope is not a strategy.
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Ergo, hoping or planning for herd immunity not generated by a vaccine is not wise at this time.
In general, I agree with your logic. OTOH, hypotheses and hope are both important things to have.
I'm kind of glad that somebody is trying for herd immunity and testing if it works. IMNSHO, Sweden is basically taking one for the team here and performing a large scale experiment (admittedly, with a dangerously big sample size). At least they're getting data. I'd rather be a part of Sweden's plan (where they at lease have health care) than, let's say, West Virginia's... but I'm happier to be in neither.