But experts are cautioning that is too soon to raise alarm over the variant dubbed Omicron XE, which is a combination of the highly transmissible BA.1 and BA.2 variants, the latter of which is suspected to be behind a resurgence in cases in Europe and an uptick in the United States.
Only a handful of XE cases have been detected outside of the UK. And there is no evidence indicating recombinant variants are any more of a public health threat than other variants, but experts say they should be closely monitored and are likely to become more frequent.
Here is what we know so far about XE.
What do early estimates tell us about XE?
XE is a hybrid of two strains of Omicron: BA.1 and BA.2, which has become the dominant strain across the United States and is referred to as “stealth Omicron.” The recombinant variant was first detected in the United Kingdom on Jan. 19 via sequencing.
While early growth rates for XE were not significantly different from BA.2, the United Kingdom Health Security Agency said, data collected from mid-January up to mid-March demonstrated that it is approximately 9.8 percent more transmissible than BA.2.
But in its latest report, the UKHSA documented that the number of XE cases in England had risen from 637 identified in March to 1,125 as of April 5. The UKHSA also said data compiled over the most recent three-week period indicated XE may be about 21 percent more transmissible than BA.2.
“This particular recombinant, XE, has shown a variable growth rate and we cannot yet confirm whether it has a true growth advantage,” said Susan Hopkins, the chief medical advisor of the UKHSA, in a statement. “So far there is not enough evidence to draw conclusions about transmissibility, severity, or vaccine effectiveness.”
What is a recombinant variant?
A recombinant variant occurs when an individual becomes infected with two or more variants at the same time, according to a recent report from the UKHSA. The event results in a mixing of the genetic material within their body. This is not an unusual occurrence — in fact, Grace Roberts, a virologist with the University of Leeds, wrote in The Conversation that this happens regularly with other viruses such as influenza and HIV. Several other recombinant variants of the coronavirus have also been detected during the pandemic thus far.
The vast majority of recombinant variants do not “confer any advantage to the virus and die out relatively quickly,” the UKHSA said.
Where have cases of XE been identified?
The vast majority of XE cases have been identified in the United Kingdom, with 1,125 cases reported in England and 1,179 sequences detected in the UK data as of April 5, according to the latest UKHSA report.
A limited number of XE cases have also been reported in other countries including China, Thailand, India, and Israel. Japan became the latest country to announce on Monday that it had for the first time detected XE, which occurred during a routine check at an airport, The Japan Times reported. The variant was identified in a vaccinated woman who traveled from the United States at the end of March and tested positive for the virus upon arrival.
Should we be worried about XE?
Experts say that at this moment, there is no need to be overly concerned about XE. But a group of researchers stressed in a piece for The Conversation that recombinant variants should be closely monitored to determine if they induce changes to the transmissibility of the virus, impact the disease severity, or are able to evade vaccine-induced immune protection. No evidence so far suggests recombinants pose any more risk than other variants.
What else should we watch out for?
In addition to XE, the UKHSA is also currently monitoring two other recombinants: XF and XD. While XE is a recombinant of Omicron BA.1 and BA.2, both XF and XD are recombinants of Delta and Omicron BA.1, the UKHSA said.
The WHO wrote in its latest report that it will continue to monitor circulating recombinant forms of BA.1 and BA.2, which includes XE and XD. Like the variants BA.1 and BA.2, XE has also not yet been classified as a variant of concern by the WHO, which assigns Greek letters to those that have been identified as such. It does so after it has been determined that the variant has demonstrated one or more changes “at a degree of global public health significance.”
Meanwhile, state health officials in New York announced on Wednesday that two new Omicron subvariants — called BA.2.12 and BA.2.12.1 — may be responsible for rising infections in the region over the past several weeks. It is the first reported instance of “significant community spread due to the new subvariants” in the country, but officials said “there is no evidence of increased disease severity by these subvariants” at this time.