Mu COVID variant: All you need to know about the new UK coronavirus strain © Getty Images

Mu COVID variant: All you need to know about the new UK coronavirus strain

The Mu coronavirus strain continues to be monitored as a variant of interest. Here's what we know about the strain.

The UK has reported no new cases of the Mu variant of interest within the week leading up to 8 September 2021. The variant is thought to have been responsible for 53 cases of coronavirus in the UK, according to Government data.


However, some scientists warn that the Mu strain, also known as B.1.621, could be ‘highly resistant’ to antibodies. This finding comes from a pre-print paper, so is not conclusive and has not been peer-reviewed.

In the pre-print paper, the scientists write that when comparing different SARS-CoV-2 spike proteins, those found on the Mu variant were more resistant to neutralisation in lab-made samples of COVID antibodies than all other currently recognised variants of interest and concern – including the Delta variant currently dominating in the UK.

This would mean that vaccinated individuals and those who’ve previously had COVID could have less protection against this variant compared with other strains.

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“The Mu variant was first reported in Colombia in January,” Dr Paúl Cárdenas, a specialist in infectious diseases, told BBC Science Focus. “We found it in May in Ecuador. It caught our attention as it has several mutations similar to Beta, and some with Iota, Delta, Alpha and Eta.”

Cárdenas says that within a few weeks the Mu strain had outcompeted the variants most common in Ecuador prior to Mu’s arrival.

“It has been found in many other countries of South America including Peru, Uruguay, Bolivia, Paraguay and Uruguay, and there have been small outbreaks from this variant in Spain, France and Florida, USA.”

Why has Mu been designated a variant of interest?

The Mu strain carries several mutation to the spike gene.

An article published in the Journal of Medical Virology states that  some of Mu’s mutations are found in other variants of concern, while others are new. The experts, based in Rome, studied the spread of Mu in Italy. They say that the strain has mutations that are also found in the variants of concern Alpha, Beta and Gamma.

According to the European Centre for Disease Prevention and Control, the Mu variant also carries a mutation which is found in the Delta variant.

A map of occurrences of the B.1.621 Mu variant of coronavirus, as of 17 September 2021 © cov-lineages.or
A map of occurrences of the B.1.621 Mu variant of coronavirus, as of 17 September 2021 © cov-lineages.or

The WHO’s variant of interest designation means the strain has:

  • genetic changes that are predicted or known to affect virus characteristics such as transmissibility, disease severity, immune escape, diagnostic or therapeutic escape
  • been identified to cause significant community transmission or multiple COVID-19 clusters, in multiple countries with increasing relative prevalence alongside increasing number of cases over time
  • may have other apparent epidemiological impacts to suggest an emerging risk to global public health.

How many cases of the Mu variant have been detected in the UK?

According to UK Government data, there have been a total of 53 cases of the Mu COVID variant in the UK. This is broken down as 47 cases in England, 3 in Scotland, 2 in Northern Ireland and 1 in Wales.

The strain has been called VUI-21JUL-01 by Public Health England.

Will vaccines still work 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.

The lab-based study by experts in Rome tested the effectiveness of the BioNTech-Pfizer vaccine against the Mu variant, and found that “despite several mutations in Spike, SARS-CoV-2 B.1.621 is neutralised by the [Pfizer] vaccine-elicited antibodies”. They note that the neutralisation, though effective, was lower than that of other coronavirus variants.

They say this highlights “the importance of properly quarantining people after abroad travel to avoid spreading of newly emerging SARS-CoV-2 lineages to different countries”.

However, a study published in The Lancet Infectious Diseases on 13 August 2021 suggests that the Mu variant has “two cases of a potential vaccine-escape”.

They write that several of the spike mutations within Mu “have been reported to show reduced neutralisation by antibodies”.

The Mu variant was also found to have the same spike mutation that has been associated with a weakened vaccine response in the Beta variant.

“The presence of mutations associated with vaccine escape might warrant reclassification of this variant to a variant of concern.”

Is the Mu variant more transmissible?

There are currently very few studies looking into the Mu variant. As research on this specific variant is all in early stages, it’s too early to know whether the strain is more transmissible or more severe than other variants.

One pre-print study – which is yet to be peer-reviewed – says that although the Mu strain is not yet classified as a ‘variant of concern’ by WHO (which means it is being more closely monitored than a variant of interest), the mutations found in the B.1.621 strain could have “a synergistic impact on attributes such as reduction of vaccine-induced protection from severe disease, increased transmission and disease severity”.

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 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

The NHS say that most people who have symptoms of COVID-19 will have at least one of the above.

What other variants have been identified in the UK?

As of 17 September 2021, there are five strains considered to be ‘of concern’ by PHE, including the Delta and Alpha variants.

The PHE are monitoring a total of 16 variants.


BBC Science Focus has contacted PHE for a comment on the spread of Mu.

About our expert, Dr Paúl Cárdenas

Dr Cárdenas is an associate professor at the Institute of Microbiology in the University of San Francisco de Quito. He has a PhD in Genomic Medicine from Imperial College London, and has published several papers thoroughout the pandemic on the detection and spread of COVID variants.