South African (SA) scientists were among others in Africa who diagnosed the Omicron Covid virus in the past. Now there is a new variant, just as scientists are getting to understand the alphabet soup of SARS-CoV-2 variants circulating around the globe.
Great-grandchild of Omicron
As its name suggests, XBB.1.5 is an offshoot of a SARS-CoV-2 variant called XBB. That lineage is a recombinant of two descendants of the BA.2 lineage that began spiking in early 2022; BA.2 itself is an offshoot of Omicron. XBB’s spike protein has a suite of mutations that boost the variant’s ability to evade antibodies. This has helped XBB to become common over the past few months, particularly in Asia, where it caused a surge in cases in Singapore.
While the XBB.1.5 sub variant is currently responsible for 28% of all Covid-19 cases in America and its prevalence is expected to rise worldwide, SA scientists say that while this “peculiar mutation” had been found in the country, there is “no big wave. According to BusinessTech, the new variant is being closely monitored by the SA Health Department, but currently there are no new restrictions in place.
“It’s almost certainly going to dominate in the world. I cannot find a single competitor now. Everything else is incomparable,” says Yunlong Cao, an immunologist at Peking University in Beijing whose team is studying the properties of XBB.1.5 in the laboratory.
Not Enough Data Yet
Scientists caution that XBB.1.5’s impact is still far from clear. The variant might not cause a big surge in infections or hospitalisations in many countries, thanks to high immunity built up from exposure to earlier waves of COVID-19 and vaccinations, particularly recent boosters for individuals most at risk of severe disease.
Variant-watchers noticed XBB.1.5 in late 2022, thanks to a rarely seen amino-acid change, called F486P, in the spike protein. Experiments from Cao’s lab suggest that the mutation improves the variant’s ability to attach to the human ACE2 receptor, which SARS-CoV-2 uses to invade cells1. The results were posted to the bioRxiv preprint server on 5 January and have not yet been peer reviewed.
The relationship between a variant’s ability to attach to ACE2 and its transmissibility isn’t fully clear according says Jesse Bloom, an evolutionary virologist at the Fred Hutchinson Cancer Centre in Seattle, Washington. However, with XBB.1.5, F486P seems to have given it boost enabling the virus to spread. It is also being labelled as the most transmittable variant so far.
Tulio de Oliveira, a bio-informatician at Stellenbosch University in South Africa, thinks researchers should look at hospital cases and other measures of disease severity to best measure XBB.1.5’s impact. Factors such as a cold snap in the North-eastern United States and holiday gatherings could partly explain the variant’s apparent surge, he says.
“I think that many scientists are jumping to conclusions and predictions very early and with very little data.” – Tulio de Oliveira
Jennifer Surtees, a biochemist at the University at Buffalo in New York, wonders whether researchers are overestimating XBB.1.5’s growth in the North-eastern United States. The variant has become more common in the western New York sequences her team handles, but she hasn’t yet noticed the meteoric rise in XBB.1.5 genomes that labs in New York City are recording.
Gauging XBB.1.5’s impact might not be straightforward, owing to the drop-off in testing for COVID-19, Surtees adds. “I think that we are truly flying blind right now. We have no idea how many cases are really out there.”
One thing that researchers can agree on is that XBB.1.5, like its predecessor XBB, is a master of immune evasion. It carries numerous spike mutations that blunt the potency of antibodies raised by vaccination and infection — including earlier Omicron strains. Bivalent vaccines boost levels of antibodies capable of blocking XBB infection (and probably XBB.1.5) in lab tests, but not by much.
Throughout 2022, researchers watched Omicron lineages pick up a succession of antibody-evading mutations in the viral spike protein that allowed new lineages to overcome immunity gained from vaccines and previous waves. XBB.1.5 is vastly more transmissible than other circulating variants thanks to the addition of the F486P mutation, so there is currently little evolutionary pressure on the lineage to change further.
But as global immunity to the sub variant builds, XBB.1.5 won’t stand still, it is predicted that we are going to see a lot of new mutations that we have never seen before.