What is 'Centaurus', the Omicron sub-variant that has health authorities on alert?

With Australia's COVID-19 cases rising, most people have heard of the BA.4 and BA.5 Omicron sub-variants which are driving the new wave of infections.

However, there's another form of the virus which has caught the attention of health authorities across the globe, including the World Health Organisation (WHO): BA.2.75 or, as it was called on Twitter, Centaurus.

Here's everything you need to know about the sub-variant.

READ MORE: Catching COVID-19 linked to sudden spike in heart disease and diabetes risk, study finds

What is Centaurus?

Like BA.4 and BA.5, Centaurus is a more transmissible sub-variant of the Omicron strain of COVID-19.

It has been found in Australia and a number of other countries, but it is currently most prevalent in India.

Centaurus was first detected in May, but it wasn't until earlier this month that it started to create more concern – WHO director-general Tedros Adhanom Ghebreyesus said the organisation was tracking it on July 6.

EXPLAINED: Australians aged over 30 now eligible for a fourth COVID-19 vaccine dose - Everything you need to know

How is it different from the original Omicron variant?

Centaurus contains the same mutations that the BA.2 strain contained, plus some additional ones on top of that – hence the BA.2.75 designation.

That large number of mutations separating the new variant from Omicron predecessors is what's fuelling experts' concerns. Some of those mutations are in areas that relate to the spike protein and could allow the virus to bind onto cells more efficiently.

Another concern is that the genetic tweaks may make it easier for the virus to skirt past antibodies – protective proteins made by the body in response to a vaccine or infection from an earlier variant – making it more transmissible.

"It's still really early on for us to draw too many conclusions," US virology expert Matthew Binnicker, the director of clinical virology at the Mayo Clinic in Rochester, Minnesota, said.

"But it does look like... the rates of transmission are showing kind of that exponential increase."

EXPLAINED: What are the current mask rules in Australia?

But even though Centaurus is thought to be highly contagious and capable of dodging vaccines and immunity from previous infections, it is still unknown if it causes more serious disease than other variants, and it may still be less infectious than BA.4 and BA.5.

"A recent study suggests that it is less immune evasive than BA.4/5 with previous Omicron infection and vaccination," Dr Benjamin Schwessinger from the Australian National University said.

"At the same time the study found that it is more immune evasive in individuals with prior Delta infection. Hence the growth advantage of BA.2.75 might very well be population dependent."

EXPLAINED: COVID-19 isolation payments are back - what you need to know

Do vaccines still work against it?

While data about the new sub-variant is limited, the shorter answer is "yes" – experts say vaccines and boosters are still the best defence against severe COVID-19.

"The COVID-19 vaccines that are in use right now, which are based on the ancestral strain (the original form of the virus from late 2019), are very effective at preventing severe diseases and death, including against BA.4 and BA.5 and other sublineages of Omicron," Van Kerkhove said on Saturday.

That message was echoed by Binnicker.

"Some may say, 'Well, vaccination and boosting hasn't prevented people from getting infected.' And, yes, that is true," he said.

"But what we have seen is that the rates of people ending up in the hospital and dying have significantly decreased. As more people have been vaccinated, boosted or naturally infected, we are starting to see the background levels of immunity worldwide creep up."

EXPLAINED: What is Marburg virus

Where has it been found?

Centaurus was first detected in India in May.

It has now been spotted in several distant states in the country, and appears to be spreading faster than other variants there, said Lipi Thukral, a scientist at the Council of Scientific and Industrial Research-Institute of Genomics and Integrative Biology in New Delhi.

It's also been detected in 13 other countries, including Australia. In its most recent weekly update, NSW Health said three cases of Centaurus had been detected in returning travellers.

"NSW Health is monitoring the international situation in regard to the BA.2.75 subvariant which has recently emerged," it said in a statement.

"Three cases have been reported in returned travellers in NSW which have been confirmed by whole genome sequencing."

Other countries where it has been found include Germany, the United Kingdom, Canada and the USA.

However, it is difficult to track exactly how prevalent it is and how quickly it's spreading, as the increase in at-home rapid antigen testing – and accompanying decrease in PCR testing – means there is less data for authorities to work with.

In her press conference on Saturday, Van Kerkhove said there are roughly 200 sequences of BA.2.75 available from across the globe.

"Our understanding of this virus is quite limited so far because we have very few sequences available," she said.

READ MORE: NSW records 'three times' more flu cases than any other Australian state

How did it get its name?

Unlike Omicron, Delta and the other variants of concern, Centaurus is not a letter of the Greek alphabet.

That's because it's a sub-variant of Omicron, and so is still a form of that particular strain. The WHO doesn't name sub-variants after Greek letters like they do with variants of concern.

Instead, we have Twitter to thank for the unofficial moniker, after a user on the social media platform decided to name it after the Centaurus A galaxy.

EXPLAINED: What is foot and mouth disease and can humans get it?

- Additional reporting with Associated Press, CNN