The World Health Organization has added another version of coronavirus to its list of “variants of interest” amid concerns that it may partially evade the immunity people have developed from past infection or vaccination.
The Mu variant, also known as B.1.621, was added to the WHO’s watchlist on 30 August after it was detected in 39 countries and found to possess a cluster of mutations that may make it less susceptible to the immune protection many have acquired.
According to the WHO’s weekly bulletin on the pandemic, the Mu variant “has a constellation of mutations that indicate potential properties of immune escape”. Preliminary data suggests it may evade immune defences in a similar way to the Beta variant first discovered in South Africa, the report adds, but this needs to be confirmed by further work.
The Mu variant was first identified in Colombia in January 2021. Since then, sporadic cases and some larger outbreaks have been recorded around the world. Beyond South America, cases have been reported in the UK, Europe, the US and Hong Kong. While the variant makes up less than 0.1% of Covid infections globally, it may be gaining ground in Colombia and Ecuador where it accounts for 39% and 13% of Covid cases respectively.
Scientists and public health officials are particularly eager to know whether the Mu variant is more transmissible, or causes more serious disease, than the Delta variant that is dominant in much of the world. “The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes,” the WHO bulletin states.
At least 32 cases of the Mu variant have been detected in the UK, where the pattern of infections suggests it was brought in by travellers on multiple occasions. A report by Public Health England (PHE) in July said most were found in London and in people in their 20s. Some of those testing positive for Mu had received one or two doses of Covid vaccine.
The Mu variant was added to PHE’s list of variants under investigation in July. The designation, which refers to Mu as VUI-21JUL-01, means the variant will be monitored to see how it behaves. So far it has not raised alarm as much as Alpha and Delta, which are classified as more serious variants of concern, largely because of their increased transmissibility but also concerns about evading immune defences.
A risk assessment of the Mu variant released by PHE in August highlighted laboratory work that suggests the variant is at least as resistant as the Beta variant to immunity arising from vaccination. But more evidence is needed from other laboratory studies and real-world cases of the variant. How much of a threat the variant poses is highly uncertain and depends on whether cases grow substantially in the weeks and months ahead, particularly in the presence of the fast-spreading Delta variant.
“At present, there is no evidence that VUI-21JUL-01 is outcompeting the Delta variant and it appears unlikely that it is more transmissible,” the report states, though it goes on to warn: “Immune escape may contribute to future changes in growth.”
Part of the concern about Mu comes from the particular mutations it carries. One genetic change, the P681H mutation, is found in the Alpha variant first detected in Kent and has been linked to faster transmission. Other mutations, including E484K and K417N, may help the virus evade immunity defences, which could give the variant an advantage over Delta as immunity rises into the autumn.