Almost from the moment it made the jump to humans, the SARS-CoV-2 virus has been picking up mutations and creating new lineages as it expands into different populations. In practical terms, the vast majority of these mutations makes absolutely no difference; the resulting virus has the same properties as the unmutated form it’s derived from.

But there have been a number of cases where variants surge in frequency. Early on in the pandemic, this was often the product of the variant moving into a previously unexposed population—a matter of chance rather than a feature of the virus. Separating out these cases from instances where mutations make the virus more dangerous is a serious challenge. But this week, an international team of researchers has published evidence showing that a variant first characterized in Brazil is likely to represent a significant additional threat.

There’s a lot of uncertainty about the details, but the virus appears to be more infectious and more likely to infect those who have immunity to other viral strains, and it might even be more lethal. And, as of when the paper was written, the lineage had been detected in over 35 countries.