But it also gets an edge from mutations that let it escape antibodies from other coronaviruses. They estimate that in 100 people who were infected with non-P.1 lineages in Manaus last year, somewhere between 25 and 61 of them could have been reinfected if they were exposed to P.1 in Manaus.

The researchers found support for this conclusion in an experiment in which they mixed P.1 viruses with antibodies from Brazilians who had Covid-19 last year. They found that the effectiveness of their antibodies dropped sixfold against P.1 compared with other coronaviruses. That drop might mean that at least some people would be vulnerable to new infections from P.1.

“There seems to be an increasing body of evidence that suggests that most of the cases associated with the second wave are indeed sort of reinfections,” Dr. Faria said.