The World Health Organization (WHO) 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 Guardian reported.

The Mu variant, also known as B.1.621, was added to the WHO’s watchlist on Monday 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 defenses 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.

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.

Part of the concern about Mu comes from the particular mutations it carries.