What is the XE Omicron hybrid and should we be worried about it?

The first time one was reportedly spotted turned out to be a false alarm.

This time it appears to be for real.

Researchers in several countries have identified hybrid variants of the Delta and Omicron strains of the coronavirus.

They have also found one recombinant variant of sublineages of Omicron, which has shown early signs of being even more transmissible than the parent variants.

Here is what we know about these variants so far.

Britain’s Health Security Agency is monitoring three recombinant lineages: XD, XE and XF.

The XD is the hybrid of Delta and BA.1, a sublineage of the Omicron variant and has been found mostly in France, Denmark and Belgium.

The XE is recombinant of two sublineages of the Omicron variants, BA.1 and BA.2, but it has three mutations that are not present in their parent sequences. It has been found in Britain only.

XF is the recombinant between Delta and BA.1 but only been detected in Britain so far.

XD was first detected in December and by March 22, 49 samples had been found in France, Denmark and Belgium.

Virologist Tom Peacock from Imperial College London said this was the recombinant to watch because of its spread into multiple countries and its inclusion of Delta, a variant that causes more severe clinical outcomes.

“The concern would be if there are non-structural changes in Delta that contribute to severity and/or cell replication,” he tweeted.

By March 22, the HSA had identified 637 XE cases in Britain. These were geographically distributed across England and increasing in number, indicating community spread.

The agency compared genome samples of the BA.2 subvariant of Omicron, the dominant variant in Britain, and found that XE was 9.8 per cent more transmissible. The agency cautioned the data might be biased in the early stages and the growth rate would converge towards an estimate of its inherent transmissibility over time.

Professor Susan Hopkins, the agency’s chief medical adviser, said XE had shown a variable growth rate but the agency could not yet confirm whether it had a true growth advantage.

“So far there is not enough evidence to draw conclusions about transmissibility, severity or vaccine effectiveness,” she said.

So far, only 39 sequences of the XF hybrid variant have been identified and only in Britain. The first was registered on January 7, and no samples have been found since February 14. The agency concluded that XF was unlikely to be associated with sustained community growth.

Yes, variants have recombined before in this pandemic. Two related viruses infecting the same cell can result in a mixed genome.

Researchers reported genomic recombination between the Delta and Alpha variants in 26 Sars-CoV-2 patient samples in Japan and between the Alpha variant and B.1.177 variants, first reported in Spain in the summer of 2020.

Hopkins, from the HSA, said recombinant variants were not unusual, particularly when there were several variants in circulation.

“Several have been identified over the course of the pandemic to date. As with other kinds of variants, most will die off relatively quickly,” she said.

Peacock, from Imperial College London, said such variants were identified by standard PCR tests and should be picked up by rapid self-testing kits too.

“I would be surprised if lateral flows didn’t work fine on them,” he said.

It is still too early to know for sure but scientists are monitoring the data closely.