But that’s likely only part of the picture. The second factor is the virus itself. Several newsworthy variants exist, including those in the U.K., South Africa, and California. (So it’s easy to lose track, as much alarm has been raised over their various mutations.) But the variant in Brazil, known as the P.1 (or B.1.1.248) lineage, has a potent combination of mutations. Not only does this variant seem to be more transmissible; its lineage carries mutations that help it escape the antibodies that we develop in response to older lineages of the coronavirus. That is, it at least has a capacity to infect people who have already recovered from COVID-19, even if their defenses protect them against other versions of the virus. One case of reinfection with this new lineage has already been documented, even though very little genomic sequencing is being done.

The mutations that help the virus spread and evade immune responses have arisen independently in multiple places. Combined with waning immunity, these factors underscore the challenge before the world: Populations may still be vulnerable to disaster scenarios just when things seem to be getting better. It’s not yet known how many of the people currently infected in Manaus have previously recovered from COVID-19. Early data suggest that the P.1 variant is now dominant in the city, but this does not mean the variant will take over everywhere. Each place and population is unique, and susceptibility will vary based on which variants have already spread. Still, the virus’s capacity to cause such a deadly second surge in Brazil suggests a dangerous evolutionary potential.