The Brazilian city of Manaus has been dealing with a raging outbreak of COVID-19, one that has left hospitals overrun and oxygen supplies dwindling; and not for the first time.The city was hit so hard by the coronavirus in the spring, that researchers estimated that 75 per cent of the population had been infected, which makes the severity of this recent outbreak unexpected and concerning.Today, Atlantic writer and Yale University public health policy lecturer Dr. James Hamblin explains the potential threat posed by new variants of the coronavirus popping up around the world, how they could upend expectations about herd immunity, and the need for a truly global vaccination response.