Evidence is mounting that Omicron’s new sister variant, known as BA.2, is more transmissible than the original strain but at this stage does not appear to be more vaccine-evasive.

The subvariant BA.2 is one of at least three sub-lineages of Omicron, the strain of COVID-19 first spotted in Africa in late November and now dominant around much of the world.

While rare in Australia and most other countries – the virus is found in about 2 per cent of local samples – BA.2 has started to gain a serious foothold in England, India and Denmark, where it has out-competed Omicron and now makes up most viruses sampled.

“That does seem to suggest there is an intrinsic transmissibility advantage,” said Dr Adam Wheatley, who heads a research team studying the immune response to COVID-19 at the University of Melbourne.

The high level of similarity between Omicron and BA.2 probably meant the sister-variant would not cause a huge wave of disease as did Delta and Omicron, Dr Wheatley said.

Instead, BA.2 is likely to slowly replace Omicron, much as a variant known as D614G quietly replaced the version of the virus that emerged from Wuhan in the middle of 2020.

“Nothing ominous has come out so far. There is no reason to panic,” said Professor Seshadri Vasan, who has been tracking COVID-19’s variants at the CSIRO.

Preliminary data released by Britain’s Health Security Agency earlier this week suggests a two-dose course of vaccine provides essentially no protection against catching either Omicron or BA.2

But a booster dose provides strong protection. Data on how effectively vaccines prevent serious illness from BA.2 is not yet in, although the virus’s dominance in Denmark has not stopped that country from easing restrictions. Early analysis there has not shown any worsening in hospitalisation rates.

“The take-home message is: if you are not vaccinated, please go and get that first dose. And if you are double-vaccinated, please go and book a booster,” Professor Vasan said.

Just 34.6 per cent of Australians aged over 12 have had a booster, leaving much of the population vulnerable to Omicron and BA.2.

Evidence from a small number of Omicron infections tracked by British health authorities suggests BA.2’s “secondary attack rate” – the chance of an infected person passing the virus on to someone else in the household – is 13.4 per cent, compared with 10.3 per cent for Omicron.

Omicron may be the most recent variant, but it did not evolve from Delta – the two viruses have family trees that diverged in 2020. Think of them as cousins twice removed.

Scientists are focusing on three main theories to explain Omicron’s emergence.

One possibility: an unknown group of people who have been harbouring an older version of the virus, even as Delta spread around the world. Omicron may have evolved there and then emerged.

Another option: COVID-19 has spilled back from humans into animals, where it has picked up new mutations before jumping back into humans as Omicron. There is good evidence now that a range of animals can be infected with COVID-19, including mink and deer. Denmark culled millions of mink after the animals came down with the virus; late last year US scientists discovered wild deer had become a huge reservoir of COVID-19.

But the most likely option, Dr Wheatley said, was Omicron emerged from a single unlucky individual. This person may have HIV or be being treated with drugs to suppress the immune system. Because their immune system was so weak, they could have been infected with COVID-19 for months.

“You have a situation where you have a virus in a person for a long time, you have an immune system that’s not functioning, and you can get lots of mutations in one person,” said Dr Wheatley.

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