AY.4.2 (Sometimes called Delta Plus), is a new mutation of the Delta variant of COVID-19, which is being monitored in the UK and across the world.
Some context with respect to the major variants we have experienced along the way including the UK variant (B.1.1.7 or alpha), the South African variant (B.1.1351 or beta), the Brazil variant (P1 or gamma), or the India variant (B.1.617 or delta). The graphics below are in order: Alpha, Beta, Gamma, Delta. There was a fair amount of concern with respect to the South African and Brazil variants. They didn't prove to be problematic in the United States. It was the same for alpha in the U.S. (it only hit Michigan really hard and had smaller waves in a few other states). Alpha in the U.S. was hard to explain, and then Delta came out of nowhere (first appearing in India). Delta clearly had an evolutionary advantage -- head and shoulders above anything else we encountered thus far).
There really isn’t anything special here beyond what we see in other Deltas. In the US, < 0.5% are AY.4.2 at the moment, and we see rapid diversification across the whole Delta tree.
The world should be monitoring more broadly than just watching Delta Plus. A future potential threat will likely present itself similarly to Delta (assuming we would recognize its potential in the same way and at about the same speed). Right now, Delta Plus is like a tropical wave forming off the west coast of Africa. It is certainly worth monitoring, but there shouldn’t be anything we do differently now.
With Britain’s relatively high vaccination rate and expected vaccine cross protection with this variant, one wouldn’t expect a major surge in cases like with the Delta parental lineage.
So this brings us to the question of why AY.4.2 is taking off so precipitously in the UK despite their high vaccination rate, when other variants did not? An educated guess says that it’s because of one-off events (like a single soccer game) that can lead to large fluctuations in variant frequencies. The actual biology of the spike protein doesn’t look special, and so short of invoking something that is not spike-based, we would not expect it to take off globally relative to the other Deltas that are out there.
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