The first case of what's now the fastest-growing coronavirus strain in the UK has been detected in Australia, with more infections set to follow as international borders reopen.
The variant, known as AY. 4.2 or "Delta plus", was found in a case in NSW hotel quarantine. So far it's the only Covid-19 infection with that strain across the nation.
While the Delta subtype was first identified overseas in July, it now makes up about 10 per cent of virus samples taken in the UK, according to health officials. Experts say it appears to be 10 to 15 per cent more transmissible than the original Delta variant.
Australia's chief health officer Paul Kelly told reporters last week that "close vigilance" was being kept on the mutation, after a former senior US official said "urgent research" was needed.
But, Professor Kelly said, "to be clear [Delta plus] is not a 'variant of interest' or a 'variant of concern' at the moment".
Variants of interest and concern are two categories designated by the World Health Organisation (WHO). The former are variants which have genetic changes that have the potential to cause increased transmissibility or illness; the latter are variants that have been proven to do just that.
"We continue to have close vigilance of the international situation to see what experience may come from this virus; to watch out for what next variant may come from this virus," Kelly said.
"In the UK there is a lot of circulating virus there, mainly in teenagers, they have recommenced school at the moment.
"A lot of cases in teenagers and their parents, that is where the majority of the 49,000 cases I think yesterday, but very importantly, there has not been the same sort of rises we have seen in previous waves into the UK in relation to hospitalisations or deaths and that is because vaccination rollout in the UK has also been very successful."
As NSW prepares to lift its cap on fully vaccinated international returns on Monday, as well as scrapping quarantine requirements for those who've received both jabs, virologists have said the emergence of the variant was no reason to slow reopening plans.
"As long as this virus has opportunities to keep infecting people, we're going to see more variants popping up," University of Sydney virologist Dr Megan Steain told The Age.
"We can't be panicking every time we see a few more mutations."
Medical microbiologist at the Doherty Institute and member of the national Communicable Diseases Genomics Network variant of concern taskforce, Dr Norelle Sherry, said the variant is "not hugely worrying at this stage".
"The apparent increased growth rate of this virus, compared to Delta, is still much less than the differences between the Alpha and Delta variants," she explained.
"That was a much bigger jump, compared to what we're seeing between Delta and AY. 4.2."
There have been a number of variants of Delta and, confusingly, many have earned the nickname "Delta plus". That doesn't mean they're the same variant, just that they are similar derivations of Delta.
In July, a variant called AY. 1 began circulating in India and was dubbed "Delta plus".
That variant has had next to no impact on cases in India, which are now at their lowest in more than six months.
The new Delta plus has a number of mutations that may – or may not – be of concern.
Professor Francois Balloux of University College London's Genetics Institute said AY. 4.2 carried two spike mutations known as Y145H and A222V. Spike proteins help the virus enter the cells.
"Both the Y145H and A222V mutations have been found in various other SARS-CoV-2 lineages since the beginning of the pandemic, but have remained at low frequency until now," he told the BBC.
"It is potentially a marginally more infectious strain. Up to 10 per cent more transmissible.
"[But] it's nothing compared with what we saw with Alpha and Delta, which were something like 50 to 60 per cent more transmissible. So we are talking about something quite subtle here and that is currently under investigation.
"At this stage I would say wait and see, don't panic. It might be slightly, subtly more transmissible but it is not something absolutely disastrous like we saw previously."
– with Benedict Brook