A new report on Friday from the UK Health Ministry indicated the rising prevalence of a new offshoot of the Delta variant of Covid-19. The offshoot is so new that it does not have an official Pango lineage designation — such as B.1.167.2 for the original Delta variant — which is the nomenclature used by most scientists.
The new Delta sub-lineage is being labeled as AY.4.2. More commonly, it’s being called “Delta Plus.”
First identified in July of this year, per the BBC, AY.4.2 was found in 6% of the Covid-19 samples tested in the week beginning September 27, according to the UK Health Ministry, which noted the offshoot’s “increasing trajectory.”
This sublineage is currently increasing in frequency. It includes spike mutations A222V and Y145H. In the week beginning 27 September 2021 (the last week with complete sequencing data), this sublineage accounted for approximately 6% of all sequences generated, on an increasing trajectory. This estimate may be imprecise due to known sequencing issues affecting position S:145.
The BBC reports that a small number of Delta Plus cases have been identified in the U.S. and Denmark. Israel, according to another report, detected its first case of AY.4.2 today.
But Balloux warned that even an approximately 10% increase in infectiousness “does not explain much of the recent case rises in the UK. Assuming 10% higher transmissibility and a freq of 10% only translates in 1% additional cases per ~5 day viral generation interval.” The rise of cases overall in the UK since the end of July has been a staggering 85%.
But make no mistake, a 10% rise in transmissibility would be significant, especially since that’s 10% more than the more well-known Delta variant first identified in India, B.1.167.2, which itself already spreads “much faster than other variants,” according to the CDC. That would likely make AY.4.2 the most infectious Covid-19 variant documented to date.
