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, fuelling a new surge of Covid-19 cases. By the end of August, it had been detected in dozens of countries, and the WHO 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. And then it fizzled. Today, the variant has all but vanished. For every Delta or Omicron there is a Gamma, Iota or Mu, variants that drove local surges but never swept to global dominance. And while understanding Omicron remains a critical public health priority, there are lessons to be learned from these lesser lineages, experts say. “This virus has no incentive to stop adapting and evolving,” said Joel Wertheim, a molecular epidemiologist at the University of California San Diego. “And seeing how it did that in the past will help us prepare for what it might do in the future.”
Studies of the also-rans have shed light on surveillance gaps and policy blunders — providing more evidence that America’s international travel bans were not effective — and on what makes the virus successful, suggesting that in the early phase of the pandemic, transmissibility was more important than immune evasion. The research also highlights how much context matters; variants that make an impact in some places never gain a foothold in others. As a result, predicting which variants will surge to dominance is difficult, and staying on top of future variants and pathogens will require comprehensive, nearly real-time surveillance.
“We can gain a lot by looking at the viral genomic sequence and saying, ‘This one is probably worse than another one,’” Dr. Wertheim said. “But the only way to really know is to watch it spread, because there are a whole lot of potentially dangerous variants that never took hold.”
The coronavirus is constantly changing, and most new variants never get noticed or named. But others raise alarms, either because they quickly become more common or because their genomes look ominous. Both were true of Mu as it spread in Colombia. “It contained a couple of mutations that people had been watching very closely,” said Mary Petrone, a genomic epidemiologist at the University of Sydney and an author of the new Mu paper. Several of the mutations in its spike protein had been documented in other immune-evasive variants, including Beta and Gamma.
In the new study, which has not yet been published in a scientific journal, scientists compared Mu’s biological characteristics to those of Alpha, Beta, Delta, Gamma and the original virus. Mu did not replicate faster than any other variant, they found, but it was the most immune-evasive of the bunch — more resistant to antibodies than any known variant besides Omicron, Dr. Fauver said.
By analysing the genomic sequences of Mu samples collected from all over the world, the researchers reconstructed the variant’s spread. They concluded that Mu had likely emerged in South America in mid-2020. It then circulated for months before it was detected. Genomic surveillance in many parts of South America was “patchy and incomplete,” said Jesse Bloom, an expert in viral evolution at the Fred Hutchinson Cancer Research Center in Seattle. “If there had been better surveillance in those regions, possibly it would have been easier to make a faster assessment of how worried to be about Mu.”