Summary: This is concerning because it may mean that people who were previously immune (had COVID) are no longer immune or a new strain is infecting people even though they had immunity to the original strain. Either of which, if true, would be bad.<p>Data is preliminary but based on previous metrics on exposure and “immunity” we shouldn’t see so many people getting sick. Also possible this is another “super contagious” strain and it is ripping its way across the remaining population that’s not immune. Also possible they just overestimated the number of people that should be immune (obviously this would be the ideal answer).<p>Everyone is sort of holding their breath that the vaccines will maintain immunity until “herd immunity” can be established. If the virus mutates so existing immunity no longer protects you then we’re sort of back to square one and this will be like fighting the flu where it never “goes away.”
I've seen a few explanations of this that aren't as doom and gloom as some of the media reports. The first is that the seroprevalence data is based on blood donors, which is not a random sample. It looks like they tried to control for that in the cited study, but I don't see an exact breakdown.<p>Secondly, Manaus is apparently a hospital the serves a pretty vast rural area, so the hospitalizations may be catching a lot of people that would have fallen outside of the immediate area covered by the seroprevalence survey.
> ...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.<p>> There are at least four non-mutually exclusive possible explanations for the resurgence of COVID-19 in Manaus.<p>1. wrong case reproduction number (R0) of 3<p>2. waning immunity<p>3. a new immunity evading variant<p>4. a more transmissible variant with a higher effective reproductive number<p>I'd say the simplest explanation is that the 76% seroprevalence study was wrong.