The Mu variant (also known as B.1.621) is the latest form of SARS-CoV-2 to be designated a “Variant of Interest” by the World Health Organization. The agency announced the designation had been made in last week’s epidemiological update, citing an uptick in Mu infections in Ecuador and Colombia, where the variant was first identified in January. In Colombia, it underlies nearly 40% of cases, according to WHO data.

According to Public Health England, the variant shares mutations with other variants, most notably the Beta (B.1.351) variant first identified in South Africa. These include the E484K and K417N mutations, which researchers have linked to immune escape. K417N is also seen in the “Delta plus” variant. Additionally, Mu has the P681H mutation seen in the Alpha (B.1.1.7) variant, which is associated with increased transmissibility.

Detailed studies of the Mu variant’s characteristics have yet to be conducted, but it exhibited Beta-like escape of vaccine-induced immune protection in preliminary data presented to the WHO’s Virus Evolution Working Group, UN News reports. Infectious disease researcher Paúl Cárdenas with Universidad San Francisco de Quito in Ecuador tells The Washington Post that he has studied Mu and that the data suggest the variant is more transmissible than the original SARS-CoV-2, adding that Mu has outcompeted Gamma and Alpha in Ecuador and Colombia.

Still, vaccine developers appear confident. “To date, we are encouraged by both the real-world data and laboratory studies of the vaccine and see no evidence that the virus or circulating variants of concern regularly escape protection,” Pfizer spokesperson Kit Longley tells the Post via email.

In its update, the WHO called for more research on the Mu variant, including how it may interact with the highly contagious Delta (B.1.617) variant.

According to Outbreak.info, a data aggregation site maintained by three Scripps Research labs, Mu has been detected in at least 46 countries as of September 7, including the United States, where Delta continues to dominate but more than 2,100 cases of Mu have been reported. Mu has been detected in every US state except Nebraska. As of September 7, the site shows California leading US Mu cases with 399 cases reported, followed by Florida (305 cases) and New York (203 cases). In Alaska (147 cases), the variant accounts for about 4% of infections.  

The New York Times reports that Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, says US officials are keeping a close eye on the Mu variant. “We take everything like that seriously, but we don’t consider it an immediate threat right now.”