In new data from lab experiments in Japan, the researchers found BA.2 was more transmissible, vaccine-resistant and liable to cause severe disease than Omicron, adding it also appeared resistant to several types of COVID-19 treatment.

"Based on our findings, we propose that BA.2 should be recognised as a unique variant of concern," the researchers said.

The new research, which is yet to be peer reviewed, coincides with the reopening of international borders in Victoria and growing calls to further ease restrictions in the face of plateauing case numbers and declining rates of hospitalisation.

But with a little over half the adult population in the state boosted, including in Ballarat, epidemiologists have sounded a warning against vaccine complacency, noting the clear potential carried by BA.2 to give rise to a prolonged wave of deadly infections.

What is the BA.2 subvariant?

The BA.2 subvariant evolved from the Omicron lineage of the virus, which was itself an offshoot of the original ancestral strain that emerged in Wuhan.

Since January, BA.2 has rapidly displaced its sister - the original Omicron - and swept to dominance in many quarters of the world, including Denmark, the Philippines, India, Singapore, Austria and South Africa, and has also been detected in Australia.

According to the most recent WHO data, the subvariant has been identified through genomic sequencing in at least 74 countries.

Is it more transmissible?

Yes. The subvariant is at least 1.5 times more contagious than the original Omicron.

Bearing in mind Omicron had a four per cent growth advantage of Delta, which itself was three times as transmissible as the Wuhan strain, BA.2 is on any view the fittest variant of COVID-19 yet.

Is BA.2 more dangerous than Omicron?

Notwithstanding early predictions the subvariant would likely be either as mild or milder still than Omicron, the most recent studies reveal BA.2 has virological characteristics which render it more severe than Omicron, potentially placing it on a par with Delta.

For instance, in the study from Japan, BA.2 - like Delta - was shown to replicate in the lungs of a host at a far more rapid pace than the original Omicron, thereby giving the virus greater scope to inflict more damage to a person's lungs.

The study's conclusion BA.2 is more virulent than Omicron corresponds with real world data in Denmark, where hospitalisations and deaths have been climbing since BA.2 superseded Omicron as the dominant variant.

Leading epidemiologist Nancy Baxter, head of the school of public health at the University of Melbourne, said there was little doubt the emergence of a fitter, more pathogenic virus should occasion concern.

"There's more and more evidence that BA.2 is more severe, and given we already know it's more transmissible, it's looking worse on a number of levels than the regular Omicron," Professor Baxter said.

Are you at risk if you haven't had the booster shot?

Professor Baxter said the subvariant posed an "obvious threat" to any person who was due for their booster shot but had not yet received it.

"We know that if you've only had one or two doses and not the booster [that] your chances of transmission with the original Omicron were very, very high, so they're going to be higher still with BA.2," she said. "So, you need the booster to protect against both getting it and the very real risk of serious disease, hospitalisation and death if it's more virulent."

These views find support in recent Victorian hospital data, which showed that a person who had received a booster shot was about 4.5 times less likely than someone with just two doses to end up in hospital due to Omicron infection.

This data, it might be noted, was consistent with data dating back to late last year, which showed double vaccination offered little in the way of protection against symptomatic infection four months post a person's second dose.

Meanwhile, someone who's unvaccinated is about 34 times more likely to find themselves in ICU than a person who has received three doses.

Citing that data, the state's chief medical officer Brett Sutton recently said there was accordingly every reason to guard against vaccine complacency.

"The data is clear that getting your third dose will significantly reduce your chances of going to hospital, going to ICU or dying from COVID-19," Professor Sutton said.

Finally, BA.2 is also resistant to some COVID-19 treatments, including sotrovimab - the monoclonal antibody treatment the federal government recently purchased.

Does BA.2 evade natural immunity?

In one of the few silver linings to the summer Omicron wave, it appears prior Omicron infection does provide some protection against infection by the subvariant, particularly in vaccinated people.

Citing the explosion of cases within the community over the last month or two, Professor Baxter said it was therefore less likely BA.2 would cause a repetition of the wave experienced in recent months.

"Because we have people protected through convalescent immunity, I don't think BA.2 is going to cause another huge spike," she said. "But because it's more transmissible and probably more severe, you will have a bump in cases and hospitalisations as it takes over."

How will your booster shot hold up?

The evidence is conflicting on this point. In one recent study released by the UK Health Security Agency, protection against serious disease was shown to wane to between 70 and 80 per cent about three months after the third dose.

In contrast, the latest data from the US Centre for Disease Control and Prevention found vaccine effectiveness only waned to that level at around the four month mark. Either way, it doesn't bode particularly well, given protection against serious disease with Delta sat around 96 per cent.

When can I get a fourth dose?

ATAGI has only given the green light for a fourth dose to severely immunocompromised people.

But given what is known about the ability of the virus to escape the immunity induced by vaccination, Professor Baxter said it was likely ATAGI would "be looking into a fourth dose now".

"If I had to predict, I'd say we're at least going to be doing a fourth dose in people who are high risk, like the elderly probably before winter," she said.

For how long will the population require repeat vaccines?

It depends. For so long as low-income countries remain unvaccinated, the prospect of a new, even fitter, variant rearing its head and causing another wave of global infections remains.

On the other hand, Professor Baxter said if scientists could produce a vaccine that focused specifically on the virus's intrinsic transmissibility, we might head some way towards defeating the virus.

"We need a vaccine that focuses on [preventing] transmission, not just death or severe disease - which is what we have now," she said.

"Because if we achieved that, then we could avoid the upstream effect of infection. If you can't get COVID [because it's less transmissible], then you're not going to get sick and end up in hospital."