The World Health Organization (WHO) has elevated a new strain of the virus that causes COVID-19 to 'Variant of Interest' (VOI) status.
Mu, first detected in Colombia in January, was previously known as B.1.621. While it makes up just a fraction of cases globally - about 0.1 percent - its rapid rise in South America has experts worried. It now accounts for 39 percent of cases in Colombia and is making inroads in neighbouring Ecuador, where it's now responsible for 13 percent of cases, according to the WHO's latest weekly update.
It's been picked up in 39 countries to date. According to variant tracking site GISAID, the closest it's made it to New Zealand is Hong Kong and Hawaii.
"The Mu variant has a constellation of mutations that indicate potential properties of immune escape," the UN body said.
"Preliminary data... show a reduction in neutralisation capacity of convalescent and vaccine sera similar to that seen for the Beta variant, but this needs to be confirmed by further studies."
In other words, Mu appears to be about as good at breaking through immunity obtained from natural infection and vaccines as the SARS-CoV-2 Beta variant from South Africa, which has the higher ranking of Variant of Concern (VOC).
There are currently four VOCs - Alpha (first seen in the UK), Beta, Gamma (Brazil) and Delta (India). Behind them are VOIs - Eta (multiple countries), Iota (New York, US), Kappa (India), Lambda (Peru) and now Mu.
"They become VOIs if they display some features that potentially make them more dangerous, then VOCs if they're having a negative effect on our ability to manage the pandemic," University of Otago epidemiologist Michael Baker told Newshub earlier this week.
A few dozen Mu cases have been detected in the UK, mostly in Londoners in their 20s, Public Health England said. Researchers there said it has mutations previously found in Alpha linked to faster transmission, as well as changes which can help it evade immune defences.
Delta, which ripped through India over the new year, has become the most dominant strain worldwide and is behind the outbreak that currently has Auckland at alert level 4. It's about twice as infectious as Alpha, which hit the UK late last year, and poses about twice the hospitalisation risk as the original strain discovered in Wuhan in 2019.
"The epidemiology of the Mu variant in South America, particularly with the co-circulation of the Delta variant, will be monitored for changes, said the WHO.
Mu's elevation to VOI comes the same week South African scientists said another variant - C.1.2 - also had a "constellation" of mutations.
It increased its share of infections there from 1 to 3 percent between June and July, still well behind Delta but enough to cause worry. A case of C.1.2 was caught at the border in New Zealand in June. It has yet to receive a VOI designation from the WHO.
Nearly 10,000 people are dying every day on average at present, the WHO update said, and nearly 4.5 million confirmed cases were reported in the week to August 29. Current hotspots include Mongolia, Iran, the US, Malaysia, Georgia, the UK and Cuba.