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Updated 2 September 2021

The Mu variant of SARS-CoV-2, first identified in Columbia and now in over 39 countries worldwide, has been declared as a “variant of interest” by the World Health Organization (WHO) on August 30. 

Scientists and experts suggest the latest variant of the virus has been found to possess a cluster of mutations that may make it less susceptible to the immune protection many have acquired. Read ahead to know all about the Mu variant of COVID-19. 

B.1.621- What is the Mu variant?

The Mu variant or B.1.621 was first identified in Colombia in January 2021. There is widespread concern over the emergence of the Mu variant as infection rates are ticking up globally again, with the highly transmissible Delta variant taking hold – especially among the unvaccinated – and in regions where anti-virus measures have been relaxed.

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 its 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. More studies are required to understand the phenotypic and clinical characteristics of this variant. 

Is the Mu variant more dangerous?

The WHO’s weekly bulletin on the pandemic said the variant has mutations indicating “potential properties of immune escape”, meaning current vaccines would be less effective against it, but that more studies would be needed to examine this further.

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.

In July, the Mu variant was added to the Public Health England’s (PHE) list of “variants under investigation”. 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 the alarm as much as Alpha and Delta, which are classified as more severe variants of concern, largely because of their increased transmissibility and concerns about evading immune defences.

Where has the Mu variant been located around the world?

Even though the infectivity and transmission rate of the Mu variant hasn’t been established yet, it has been found in about 39 countries worldwide. Sporadic cases and some more significant 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.

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. In July, a report by Public Health England (PHE) said most were found in London and people in their 20s. Some of those testing positive for Mu had received one or two doses of COVID-19 vaccines.

What are the symptoms of the Mu variant?

The Mu variant appears to have the same symptoms as all other coronavirus strains. The main symptoms of COVID-19, according to the National Health Service (NHS) are:

  • a high temperature – this means you feel hot to touch on your chest or back (you do not need to measure your temperature)
  • a new, continuous cough – this means coughing a lot for more than an hour, or three or more coughing episodes in 24 hours (if you usually have a cough, it may be worse than usual)
  • a loss or change to your sense of smell or taste – this means you’ve noticed you cannot smell or taste anything, or things smell or taste different to normal

Most people who have symptoms of COVID-19 will have at least one of the above.

Are vaccines effective against the Mu variant?

There is not yet enough evidence to say conclusively whether the Mu variant will be able to evade protection from coronavirus vaccines.

A risk assessment of the Mu variant released by Public Health England (PHE) in August highlighted laboratory work that suggests the Mu 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. However, it goes on to warn: “Immune escape may contribute to future changes in growth.”

 We will update this article as new developments unfold.