There have been dozens of reported cases of the new Mu variant of coronavirus in England, new figures show.
The strain, which was first identified in Colombia in January this year, was designated a "variant under investigation" by Public Health England in July. Earlier this week, it was also named a variant "of interest" by the World Health Organisation.
There are concerns that Mu could be more vaccine resistant than previous strains, though this remains unproven.
According to a briefing released on Friday, some 47 cases of Mu - also known as B.1.621 - have now been detected in England. No deaths have yet been attributed to Mu.
The Delta variant - which originated in India - is still by far the most prevalent strain of Covid-19 in England.
What do we know about Mu?
Scientifically known as B.1.621, the World Health Organisation (WHO) designated Mu a variant of interest on 30 August when it was found to be present in 39 different countries, and because it may have properties that allow it to evade vaccination.
It is considered worthy of special monitoring but it is still less of a potential threat than the Delta or Alpha strains which are more dangerous due to their increased virulence.
Mu is not yet widespread across the globe, however its presence of 39% in Colombia and 13% in Ecuador illustrates its ability to spread.
Preliminary data suggests it could evade human immune defences in a similar way to the Beta variant, which was first discovered in South Africa. However more investigations need to be done, the WHO said.
“The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes,” a WHO bulletin issued on Friday read.
It added that the Mu variant “has a constellation of mutations that indicate potential properties of immune escape”
It said: “Since its first identification in Colombia in January 2021, there have been a few sporadic reports of cases of the Mu variant and some larger outbreaks have been reported from other countries in South America and in Europe.
"Although the global prevalence of the Mu variant among sequenced cases has declined and is currently below 0.1%, the prevalence in Colombia and Ecuador has consistently increased.
“The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes.”
On Thursday, White House chief medical advisor Dr. Anthony Fauci saud Mu was not a danger to the US.
"We're keeping a very close eye on it," Dr Fauci said, but stressed the Delta variant was still dominant.
The Mu variant, he added, "has a constellation of mutations that suggest it would evade certain antibodies...but there isn't a lot clinical data to suggest that."
"Bottom line: we're paying attention to it. We take everything like that seriously, but we don't consider it an immediate threat right now."
There are currently four coronavirus variants of concern, as deemed by the WHO, with the Alpha variant – first recorded in Kent – seen in 193 countries, Beta in 141, Gamma in 91 and Delta in 170 countries, while Mu is the fifth variant of interest.
In England, there are four current variants of concern (VOCs) and eight variants under investigation (VUIs).
All viruses naturally mutate over time, and changes can build up in the genetic code of a virus. Most of the time the changes are so small that they have little impact on the virus but when a virus mutates in a way that benefits it, for example allowing it to spread more quickly, and causes us to be concerned about changes in the way the virus might behave, it is classified as a Variant of Concern by Public Health England.