What is BA.2?
This is an off-shoot of the Omicron lineage.
It carries all the mutations of its parent, but also has a change that makes it harder to detect using PCR tests.
It has been spotted in 40 countries , including Denmark, the Philippines, India, Sweden and Singapore.
And the share of new cases caused by the variant has risen to one in 20.
Is it really harder to track?
Britain’s explosion of Omicron was fairly easy to track because of one of the variant’s specific quirks – a deletion of its spike ‘S’ gene.
It meant the strain could be detected on widely-used PCR tests without the need for lengthy laboratory analysis, known as genome sequencing.
The deletion allowed Omicron samples to stand out from other strains circulating at the time, including Delta, simply because they did not cause the same PCR ‘S-gene target failure’ effect.
But this does not appear to be the case with BA.2.
PCR tests will still spot if someone with BA.2 has Covid.
Is it more dangerous?
Early analysis suggests this sub-variant is slightly more transmissible than Omicron.
It is already the dominant strain in parts of India and the Philippines, scientists say, with cases now rising in the UK, Germany and Denmark.
But there is no evidence to suggest it is more likely to cause serious disease.
There were fears it may be able to slip past vaccine immunity more easily than Omicron, which would explain its evolutionary edge.
But an analysis by the UK Health Security Agency found two and three doses of vaccines work just as well against both strains.
A booster was found to give 70 per cent protection against symptomatic infection from BA.2, compared to 63 per cent with the original Omicron.
Should I be concerned?
Dr Tom Peacock, one of the first scientists to warn the world about Omicron, says there is no reason to be overly concerned.
The Imperial College London expert said he thinks the variant will not have a ‘substantial impact’ on the current wave.
Professor Francois Balloux, a geneticist at Imperial College London, says people not obsessed with Covid should treat it as the same disease as Omicron.
The strain has been designated as a variant under investigation by the UK Health Security Agency.
How many cases are there?
A total of 426 BA.2 cases were identified by the UKHSA by January 21, the earliest of which was dated to December 6.
Most of the cases have been spotted in London (146), followed by the South East (97).
But the true toll of BA.2 cases will be much higher, with just a small fraction of positive tests sequenced.