There could be a confounding factor here. People who got the lower doses also got less severe reactions. So they may have guessed they got the placebo and consequently were more careful to not contract covid-19. Whereas, the full dose caused more obvious reactions, which made it clear the participant didn’t get the placebo. As a result, they went out to party.
I guess it joins a long list of accidental scientific discoveries, with Penicillin and a lot more besides.