WHO Tracking New Covid Variant Named Mu, Warns It Could Be More Resistant To Vaccine

Published 2 years ago
Scientist in a protective suit holds and compares two different Coronavirus of different color in his hands. Creative image.

TOPLINE The World Health Organization said it has identified a new coronavirus “variant of interest,” named mu—also known by its scientific name, B.1.621—in its weekly pandemic update Tuesday, warning it was becoming increasingly prevalent in Colombia and Ecuador and shows signs of possible resistance to vaccines.  


Since mu was first identified in Colombia in January 2021, there have been “sporadic reports” of cases and outbreaks in South America and Europe, the WHO said. 

While the global prevalence of mu among sequenced Covid-19 cases has fallen below 0.1%, the WHO said its prevalence has “consistently increased” in Colombia and Ecuador, where it is now responsible for around 39% and 13% of infections, respectively. 


Reports on the variant’s prevalence should be “interpreted with due consideration” given the low sequencing capacity of most countries, the agency said. 

Mu has a number of mutations that suggest it could be more resistant to vaccines, the health agency warned, but said further research would be needed to confirm this.

Preliminary data show a reduced effectiveness of vaccines “similar to that seen for the beta variant,” the WHO said.  


All viruses mutate over time and most mutations have little to no impact on the virus’ behavior. The WHO keeps track of “variants of interest,” those which have mutations “predicted or known to affect virus characteristics” like transmissibility, resistance to vaccines and disease severity. Mu is the fifth variant of interest to be monitored by the WHO since March. There are also four “variants of concern” the health agency tracks—alpha, beta, gamma and delta—which, in addition to meeting the criteria to be a variant of interest, have more significant changes, such as reducing the effectiveness of vaccines. The WHO introduced a naming convention concerning variants in May to avoid the stigma and consequences when a variant is named after where it was first discovered.



While wealthy countries hoard limited vaccine supplies for booster campaigns, many parts of the world have not yet had a chance to begin vaccinating. Limited vaccine coverage and the rapid spread of the coronavirus makes an ideal environment for new virus variants to emerge. The WHO said it would be monitoring “the epidemiology of the mu variant in South America, particularly with the co-circulation of the delta variant… for changes”


3,857. That’s how many genome sequences—analyzed samples of the virus taken from patients that can be used to track how it moves through the population—on GISAID, an open source genome repository, have been designated as mu in the past four weeks. The vast majority of these—1,544—have been reported in the U.S., and Colombia (844), Mexico (352) and Spain (351) also report a high number of sequences. This figure will be affected by both sequencing capacity, surveillance and the total number of cases in an area.