The number of people each sufferer could infect is believed to be 12, making it one of the world's most contagious diseases yet.
So why is BA.2 driving record case numbers, and are its symptoms more severe than earlier variants?
The National explains.
What is BA.2?
A sublineage of the Omicron family, which has three subvariants: BA.1, BA.2 and BA.3.
BA.1 was the one that originally took off and spread around the world, but BA.2 has gained a foothold now in many countries and is on the increase, leading to rises in case numbers.
One of the most interesting things about the three subvariants is that each is as different to the other as Alpha, Beta, Gamma and Delta are from each other.
The three Omicron sublineages, which were detected at the same time in South Africa, share 39 mutations. But each has some of their own. In the case of BA.1, there are 20; BA.2 has another 27, and BA.3 has 13 more.
BA.2 spreads faster than BA.1, about 30 per cent more according to estimates, which means it is easier to catch. Scientists think its extra mutations help to make it more transmissible.
“Omicron BA.2 is about 1.4 times more infectious than BA.1,” wrote former World Health Organisation scientist Prof Adrian Esterman on Twitter.
“The basic reproduction number (R0) for BA.1 is about 8.2, making R0 for BA.2 about 12.
“This makes it pretty close to measles, the most contagious disease we know about.”
The R0 of measles is between 12 and 18.
Are the symptoms more severe?
Studies in the UK and Denmark found no evidence that an infection with BA.2 is linked to a greater risk of being admitted to hospital than with BA.1. A more recent study from Qatar showed the same thing.
There had been some concern after an animal study in Japan found that BA.2 caused more severe disease in hamsters. But findings in animal studies do not always translate well to human beings.
New research from Hong Kong suggests, however, that BA.2 may be more severe in children than other variants.
They found the strain was associated with higher odds of paediatric ICU admission, mechanical ventilation and oxygen use.
“The intrinsic severity of Omicron BA.2 in children who had no past Covid-19 or vaccination is not mild,” the researchers said.
“And in fact … the Omicron BA.2 is more neuropathogenic than previous Sars-CoV-2 variants, influenza and parainfluenza viruses, resulting in more seizures.”
Although Omicron is milder in adults than Delta, BA.1 evades immunity from vaccination and other variants, making it easier for people who have been vaccinated to catch, particularly if they have not been boosted.
But BA.1 provides strong protection against BA.2, according to the WHO. So anyone who had Omicron earlier should be protected against BA.2.
Common symptoms of the variant include a cough, shortness of breath, loss of taste or smell, sore throat or hoarse voice, congestion or runny nose, and nausea or vomiting.
Why are case numbers rising?
Partly because of BA.2’s increased transmissibility, but many countries where case numbers are surging have dropped restrictions in an effort to learn to live with the virus.
Waning immunity is also believed to be playing a factor, as protection from boosters fades and many people received theirs months ago.
Case numbers have risen fast and high in the UK, but White House chief medical adviser Dr Anthony Fauci said this week he did not expect another surge right now in the US.
However, there are signs a new wave is beginning.
Wastewater surveillance at 15 sites showed an increase greater than 1,000 per cent in the virus from February 24 to March 10.
And on Thursday, New York state reported 3,875 new cases – an increase of 87 per cent from last week.
Why is it called stealth Omicron?
It gained this nickname because it was not initially clear to labs it was of the Omicron family.
PCR tests look for three genes and with BA.1 only two react, giving labs a handy shortcut to identify it.
But BA.2 does not have this missing gene. All three can be found under examination of PCR samples of BA.2, making it look like variants, such as Delta.