[sradman]

> ...in Manaus, Brazil, a study of blood donors indicated that 76% (95% CI 67–98) of the population had been infected with SARS-CoV-2 by October, 2020.

> There are at least four non-mutually exclusive possible explanations for the resurgence of COVID-19 in Manaus.

1. wrong case reproduction number (R0) of 3

2. waning immunity

3. a new immunity evading variant

4. a more transmissible variant with a higher effective reproductive number

I'd say the simplest explanation is that the 76% seroprevalence study was wrong.