The CDC announced this week that the BA.2 Omicron variant, which is reportedly 30% more transmissible than the original BA.1 Omicron strain — has become dominant among new cases sequenced in the United States. That’s a startling rise for a variant that was less than 1% of all sequences as recently as January. But, just as Americans are hearing about BA.2, there’s already a newer, even more transmissible variant on the rise.
There are actually three new variants that have been given designations. According to a recently-released report from the UK Health Services Agency, the two being called XD and XF are combinations of Delta and BA.1, or so-called “Deltacron” strains, which have been talked about for months but made no significant inroads in any country.
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XD is present in several European countries, but has not been detected in the UK, according to the report. XF caused a small cluster in the UK but has not been detected there since February 15. The variant of greater concern, it seems, is the one dubbed XE.
Like the other two new arrivals, XE is a recombinant strain, meaning it is made up of two previously-distinct variants. But it is not a Deltacron mix. XE is actually made up of the original Omicron (BA.1) and the newer Omicron (BA.2) which has taken over in the U.S.
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The World Health Organization issued a report yesterday with some preliminary findings about XE.
“The XE recombinant was first detected in the United Kingdom on 19 January and >600 sequences have been reported and confirmed since,” reads the WHO document. “Early-day estimates indicate a community growth rate advantage of ~10% as compared to BA.2, however this finding requires further confirmation.”
Further confirmation is getting more difficult by the day, according to WHO, which registered concern this week at what it calls “the recent significant reduction in SARS-CoV-2 testing by several Member States. Data are becoming progressively less representative, less timely, and less robust. This inhibits our collective ability to track where the virus is, how it is spreading and how it is evolving: information and analyses that remain critical to effectively end the acute phase of the pandemic.”
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Last week’s briefing from the UK Health Services Agency reinforces some of the WHO report’s assertions and urges caution about jumping to conclusions. One difference between the two documents is that the WHO data and analysis seems to be more recent.
From the UK HSA briefing:
XE shows evidence of community transmission within England, although it is currently less >1% of total sequenced cases. Early growth rates for XE were not significantly different from BA.2, but using the most recent data up to 16 March 2022, XE has a growth rate 9.8% above that of BA.2. As this estimate has not remained consistent as new data have been added, it cannot yet be interpreted as an estimate of growth advantage for the recombinant. Numbers were too small for the XE recombinant to be analysed by region.
To be clear, XE only accounts for a tiny fraction of cases worldwide. That may change, given that XE is thought to be about 10% more transmissible than the already more-transmissible BA.2. That means it may be roughly 43% more transmissible than the original Omicron that savaged the globe last winter.
But a new wave of infections from the now-dominant BA.2 has not materialized, even as restrictions have been eased. So hopefully the trend with XE, should it out-compete BA.2, will be similar. Only time — and good surveillance — will tell.
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