In early 2021, scientists in Colombia discovered a worrisome new coronavirus variant. This variant, eventually known as mu, had several troubling mutations that experts believed could help it evade the immune system’s defenses.

Over the following months, mu spread swiftly in Colombia, fueling a new surge of COVID-19 cases. By the end of August, it had been detected in dozens of countries, and the World Health Organization had designated it a "variant of interest.”

"Mu was starting to make some noise globally,” said Joseph Fauver, a genomic epidemiologist at the University of Nebraska Medical Center and an author of a recent study on the variant.