Global health authorities have identified two COVID-19 variants of interest in the past week:

Mu

The World Health Organization has identified the B.1.621 variant as a variant of interest (VOI), assigning it the ‘Mu’ letter from the Greek alphabet on August 30.

VOI is a lesser classification than variant of concern (VOC), which denotes a strain that is highly virulent and/or able to impinge preexisting public health measures such as vaccines, diagnostics, or therapeutics. Current VOCs include the Alpha, Beta, Gamma, and Delta variants.

Mu was first identified in Colombia in January of 2021, and has since been detected in over 39 countries, though it has generally failed to gain traction against competing strains with the exception of Colombia (39% prevalent) and Ecuador (13%), where Mu has made consistent gains.

WHO has flagged Mu as a VOI due to its “constellation of mutations that indicate potential properties of immune escape. Preliminary data… [shows] a reduction in neutralization capacity of convalescent and vaccinee sera similar to that seen for the Beta variant, but this needs to be confirmed by further studies.” (The Beta variant was originally detected in South Africa in May 2020).

This comparison to Beta in the WHO report is noteworthy due to the fact that some studies suggested Beta’s immunity escape to actually be higher than that of Delta. Fortunately, Beta’s prevalence waned as Delta’s ascended, so the question of the degree of potential vaccine escape remains a hypothetical. But newly emergent strains could in theory duplicate the same kind of mutations that made Beta so potentially disruptive to public health measures, and possibly in a way that’s much more transmissive than Beta.