Do you remember Mu?

It was a variant of concern with several troubling mutations that spread swiftly in Colombia in early 2021, fueling a surge of new cases. Today, Mu has all but vanished but it can still offer lessons for researchers that could help them understand the future of the pandemic.

“If we want to try and understand what makes variants like Omicron and Delta so successful, there’s only so much we can learn from studying them in isolation,” said my colleague Emily Anthes. “You have to also study the variants that they beat to understand what they were doing that Mu and Iota were not.”

New research has shown that Mu did not replicate faster than early variants like Alpha, Beta, Delta and Gamma, but that it was more resistant to antibodies than any known variant besides Omicron. Even so, Delta easily overtook Mu, along with several other immune-evasive variants including Beta, Gamma and Lambda, because it was so transmissible.

“Some variants are really good at spreading, and others are maybe fine at spreading, but much better at evading antibodies and our immune system defenses,” Emily said. “And at least for the first year or two years of the pandemic, transmissibility really won out.”

That may already be changing. As vaccinations and multiple waves of infection have changed the immune landscape, a highly immune-evasive variant should now have more of an edge, scientists said, which is probably part of the reason Omicron has been so successful.

Looking back at previous variants is also providing insight into what worked — and didn’t — in containing them.

Take Gamma: It was first identified in Brazil in late 2020 and then later detected in the U.S., despite a lengthy U.S. travel ban on most people coming from Brazil. Tracking the variant offers evidence that America’s travel bans were not particularly effective.

Lesser variants are also revealing our blind spots. By analyzing the genomic sequences of Mu samples collected from all over the world, researchers have reconstructed the variant’s spread and found that it circulated for months before it was detected.

“It’s a reminder that comprehensive, real-time surveillance is going to give us the best warning system for which variants pose a threat,” Emily said. Even countries that have had laudable tracking systems, like Britain, are starting to ease off and discontinue some aspect of their programs, she said. “There’s a real concern that we’re not doing enough.”