SÃO PAULO, Brazil — Hydroxychloroquine is not effective against Covid-19. No. Definitely not. But Brazilians still aren’t sure. After all, just the other day a friend’s cousin forwarded a headline on WhatsApp claiming that all I.C.U. beds in the city of Miracatu are empty because the mayor adopted President Jair Bolsonaro’s “early treatment” — consisting of hydroxychloroquine, ivermectin and azithromycin — for Covid-19. OK, Miracatu does not have a hospital. But still: How can we be sure?