A NEW CORONAVIRUS VARIANT ON THE BLOCK

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• India’s first case of the XBB.1.5 subvariant of Omicron was confirmed in Gujarat by the national genome sequencing consortium on December 31. XBB.1.5 has been driving COVID-19 cases in the U.S. Studies of the strain indicate that it is highly transmissible and evades pre-existing immunity. Yet it doesn’t seem to cause severe disease.
• But U.S. scientist Eric Topol wrote that it isn’t just superficially scary and we need to pay attention to it. The global prevalence of XBB.1.5 isn’t clear yet, although its parent strain has been detected in at least 35 countries. By December 30, XBB.1.5 accounted for 40.5% of all new cases in the U.S., up from 21.7% a week earlier.

How did the variant begin?

• 1.5 is a recombinant, which means its genome is the product of the genomes of two different strains spliced together.
• This can happen when two strains infect a person at the same time; a recombinant variant is produced as they replicate together. Recombinant strains also arise when existing recombinant strains mutate.
• Previous recombinants include XD (Delta + Omicron), XE (BA.1 + BA.2), and XBB (BA.2.10.1 + BA.2.75). The XBB strain is descended from BA.2.10.1.1 and BA.2.75.3.1.1.1. It mutated further and became XBB.1.5. XBB.1, which also descended from XBB, accounted for 14% of new cases in India around mid-December 2022.

How transmissible is it?

• On December 28, immunologist Yunlong Cao reported that XBB.1.5 is as immune-evasive as XBB.1. Both XBB and XBB.1 were more immune-evasive than BA.5.2 (its descendant BF.7 is surging in China) and in fact are the most evasive strains so far. Cao also wrote that a prior breakthrough infection by BF.7 didn’t appear to confer significant protection against an XBB.1.5 infection.
• Cao also mentioned that XBB.1.5 is better at binding to ACE2 receptors in the body than XBB or XBB.1. So XBB.1.5 is more transmissible. There is already empirical data to show that it spreads faster than BQ.1.1, the subvariant that it displaced in the U.S. as the dominant strain.
• Note, however, that XBB.1.5’s ACE2 binding affinity is comparable to that of BA.2.75, the Omicron sub-variant detected in India in May 2022. The transmissibility of XBB.1.5 is otherwise still high: it allows the virus to spread more, giving it more opportunities to mutate to more potent forms.
• On December 30, CBS News also quoted Dr. Barbara Mahon at the U.S. CDC saying, “There’s no suggestion at this point” that XBB.1.5 causes “more severe” disease. Anthony Fauci told CNBC he was taking heart from Singapore’s experience with XBB, where it became the dominant strain in October but didn’t trigger a hospitalisation surge.
• So XBB.1.5 has an ACE2 binding affinity similar to that of BA.2.75, could be more transmissible than XBB and XBB.1, and be as immune-evading as the two. Overall, it has a high growth advantage. And so far, it doesn’t appear to be able to cause severe disease. For now, much of what we know about XBB.1.5 is based on what we know about XBB. We need more studies on the former.

What are the implications for India?

• Generally speaking, “Variants would have been of some concern if the [virus’s] transmission wasn’t ongoing; however, as most settings have continuous virus transmission without clinical disease, even reduced efficacy against new variants in the real world has been compensated for by natural infections and booster doses,” said Chandrakant Lahariya, a vaccines and health-systems specialist.
• Lahariya also said research on new vaccines should continue but that it shouldn’t peg them to newer sub-variants: “by the time such vaccines are made available, a new subvariant might emerge.”
• He was also wary of the insufficient evidence to administer multiple COVID-19 boosters after the first, which could lead to original antigenic sin: a phenomenon whereby repeated boosting “saturates” the immune system and mitigates its response to future shots.
• “The original vaccines continue to be effective with marginal reduction in clinical effectiveness … against primary endpoints of moderate to severe disease.”

SOURCE: THE HINDU, THE ECONOMIC TIMES, PIB