Is Omicron variant XBB.1.5’s superior binding causing higher transmissibility? Premium

XBB.1.5 is the most transmissible variant that has been detected yet, and it has already spread to over 29 countries so far

The XBB.1.5 Omicron recombinant variant that was first detected in the U.S. (New York and Connecticut) in late October 2022 has now spread to at least 29 countries. It is rapidly increasing in number to become the dominant variant in the U.S., particularly in the northeast part of the country. It has been seen to have a growth advantage and has replaced other variants in a few countries in Europe, as well.

The XBB.1.5 variant has demonstrated a “stunning increase” in prevalence in the U.S, and went from “4% of sequences to 40% in just a few weeks”, Dr. Ashish Jha, White House COVID-19 Response Coordinator tweeted on January 4. As per CDC estimates, the variant is causing 75% of new cases in the northeast part of the U.S.

The XBB.1.5 variant is a descendant of XBB.1, which, in turn, descends from XBB. The XBB variant evolved through recombination of two descendants of the earlier Omicron BA.2 variant. If XBB and XBB.1 already had high transmissibility and higher immune escape, the XBB.1.5 variant is even more transmissible while retaining significant immune escape ability. 

A study posted on preprint server BioRxiv, which is yet to be peer-reviewed, found that both XBB.1 and XBB.1.5 “significantly” evaded convalescent plasma samples from BA.1, BA.5, and BF.7 breakthrough infections. However, XBB.1.5 displayed slightly weaker immune evasion capability compared with XBB.1.

The XBB.1.5 variant was first found in India on December 30 last year, and as per INSACOG five XBB.1.5 variants have been detected so far — three cases in Gujarat and one each in Karnataka and Rajasthan.

“XBB.1.5 is the most transmissible variant that has been detected yet,” Dr. Maria Van Kerkhove, an epidemiologist and the WHO’s technical lead on COVID-19 said during a press briefing on January 4. “The reason for the high transmissibility of XBB.1.5 are the mutations within this recombinant sub-variant of Omicron.” 

The higher transmissibility of XBB.1.5 will mean that it will be responsible for a larger fraction of COVID-19 cases in many countries and could also drive new surges. While it is too early to say if XBB.15 will have increased severity, there has been a sharp increase in hospitalisations in the U.S.