Breeding ground for variants. Much more likely to happen in such a high infection population. Then they spread to the world.
I'm not saying this would be a good idea, but Brazil is unlikely to be able to adequately vaccinate its population in the next year. This should be a consideration, as it was in March 2020 in several European countries before they adopted the chinese strategy.
Herd immunity is applicable to immunity and susceptibility in a population. https://academic.oup.com/cid/article/52/7/911/299077
That's obviously false, given that we are observing many meaningful variants without anything close to herd immunity.
Each person infected is an additional chance of a variant with antigenic escape, let alone vaccine escape.