A new hybrid variant, dubbed XE, is a combination of the highly transmissible BA1 and BA2 Omicron variants.
A machine sorts positive Covid-19 samples for genome testing. Photo: RNZ / Dom Thomas
It is not yet classified as a variant of concern by the World Health Organisation (WHO), and so does not have a greek letter assigned yet either.
But the WHO issued a report on 29 March saying indications were that XE has a 10 percent transmission advantage compared to BA.2, "however, this finding requires further confirmation".
It is being tracked as part of the Omicron variant "until significant differences in transmission and disease characteristics, including severity, may be reported".
Evolutionary virologist Jemma Geoghegan from Otago University told Morning Report if the transmissibility rate were true, it would make it the most transmissible subvariant of Omicron.
"It's basically a recombinant that was first found in the UK mid-January and it contains genetic material from both BA1 and BA2, which are the two most common subvariants of Omicron."
"XE is definitely one to watch" - Evolutionary virologist Jemma Geoghegan
But there was not enough evidence so far to draw conclusions about its transmissibility, severity or immune-evasion properties yet, Dr Geoghegan said.
"It's quite likely that this recombinant would possess similar disease characteristics to its parents, so that's BA1 and BA2."
Over the past two months, about 17 recombinants have been classified and assigned names, starting from the XA variant, she said.
"We don't actually know yet many of the characteristics and many of them will just die out and not go anywhere. XE is definitely one to watch."
Also on the radar is the XD recombinant - a hybrid of Delta and BA.1 variant of Omicron.
"The XD recombinant is being tracked as a VUM [variant under monitoring] by WHO, although its spread appears to have remained limited at present ... Currently available evidence does not suggest that it is more transmissible than other circulating variants," the WHO said in its report on 5 April.
Currently, Omicron BA2 is the dominant strain in New Zealand, which is similar to many countries abroad.
Dr Geoghegan said XE's prevalence remained small right now in comparison, with 637 cases of XE confirmed in the UK as of 25 March.
But with its apparent growth advantage, "there is a slight chance" that could cause another wave of infections.
"If it does increase in frequency and in prevalence in the population, and then if there's a difference between hospitalised cases, then that's something we can [observe] to better understand the severity.
"This requires robust genomic surveillance worldwide to track this. The WHO is urging countries to do genomic surveillance in a widespread fashion."
Yesterday, Director-General of Health Ashley Bloomfield said while XE was not yet detected here, it was inevitable new Covid-19 variants would arrive in the country.