I wanted to summarize what is known about the new XBB.1.5 variant of SARS-CoV-2, which is starting to generate a lot of interest.
(There are no new scientific results in this thread, it simply aggregates previously reported results for those not following topic closely.)
Reason people are discussing XBB.1.5 is because it’s so transmissible. Below are estimates of current Rt (measure of transmissibility) of different variants in US from @trvrb’s group.
XBB.1.5 more transmissible than other variants like BQ.1.1 that until recently dominated in US
High transmissibility means XBB.1.5 is becoming responsible for larger fraction of COVID-19 cases.
This continues below pattern of strain replacement we’ve seen over last few years of SARSCoV2 evolution.
Eg, there will always be new variants spreading, & right now it’s XBB.1.5.
A scientifically interesting aspect of XBB.1.5 is we pretty much understand what mutation made it so transmissible, the mechanism by which the mutation acts, and why it took so long for the mutation to emerge.
XBB.1.5 is a descendant of XBB.1, which descends from XBB, which evolved through recombination between two descendants of the earlier Omicron BA.2 variant (
The parental XBB and XBB.1 variants were already notable because they were fairly transmissible & had lot of antibody escape, as shown by @yunlong_cao et al & others (nature.com/articles/s4158…). However, XBB and XBB.1 were not as transmissible as XBB.1.5.
One of the sites that is mutated in the parental XBB/XBB.1 variants is 486 in the RBD.
486 has been a major site of antibody escape going back to the earliest variants (see image below from our antibody-escape calculator jbloomlab.github.io/SARS2_RBD_Ab_e…).
But while some major antibody escape sites such as 484 were fixing mutations in major variants by late 2020, it took a longer time for major variants to emerge with mutations at site 486: eg, BA.4/5 with F486V in spring 2022, and then XBB with F486S later in 2022.
It’s easy to understand why it took longer for variants to emerge at site 486: mutations at 486 reduce ACE2 affinity, so benefit they provide in antibody escape comes at cost to receptor binding. See below for our deep mutational scanning data from science.org/doi/10.1126/sc…
So variants like XBB/XBB.1 fixed mutation (F486S) that was beneficial for antibody escape but detrimental to ACE2 affinity.
In other words, they made an evolutionary tradeoff.
But as @LongDesertTrain noticed months ago, our deep mutational scanning shows one mutation at site 486 is not so bad for ACE2 affinity, especially in background of BA.2: F486P
In new study, we find dramatic differences in specificities of serum neutralizing antibodies in infants w single infection by a recent SARS-CoV-2 strain versus adults/children imprinted by an early viral strain.
As background, immune response to a virus is “imprinted” by first exposure, since later exposures to new viral strains often activate pre-existing B-cells.
For SARS-CoV-2, most people globally imprinted by an early viral strain from either vaccination or infection in 2020-2021.
However, small but growing fraction of population has instead been imprinted by more recent viral strain.
Specifically, we compared adults/children imprinted by original vaccine then infected w XBB* strain in 2023 vs infants only infected w XBB* in 2023.
I’ve updated SARSCoV2 antibody-escape calculator w new deep mutational scanning data of @yunlong_cao @jianfcpku
My interpretation: antigenic evolution currently constrained by pleiotropic effects of mutations on RBD-ACE2 affinity, RBD up-down position & antibody neutralization
@Nucleocapsoid @HNimanFC @mrmickme2 @0bFuSc8 @PeacockFlu @CVRHutchinson @SCOTTeHENSLEY To add to thread linked above, human British Columbia H5 case has a HA sequence (GISAID EPI_ISL_19548836) that is ambiguous at *both* site Q226 and site E190 (H3 numbering)
Both these sites play an important role in sialic acid binding specificity
@Nucleocapsoid @HNimanFC @mrmickme2 @0bFuSc8 @PeacockFlu @CVRHutchinson @SCOTTeHENSLEY If you are searching literature, these sites are E190 and Q226 in H3 numbering, E186 and Q222 in mature H5 numbering, and E202 and Q238 in sequential H5 numbering (see: )dms-vep.org/Flu_H5_America…
Here is analysis of HA mutations in H5 influenza case in Missouri resident without known contact w animals or raw milk.
TLDR: there is one HA mutation that strongly affects antigenicity, and another that merits some further study.
As background, CDC recently released partial sequence of A/Missouri/121/2024, which is virus from person in Missouri who was infected with H5 influenza.
Here I am analyzing HA protein from this release, GISAID accession EPI_ISL_19413343cdc.gov/bird-flu/spotl…
Sequence covers all of HA except signal peptide, and residues 325-351 (sequential numbering) / 312-335 (H3 numbering). The missing residues encompass HA1-HA2 boundary, and any missed mutations there unlikely to affect antigenicity or receptor binding, but could affect stability.