ECDC evaluation of the XBB.1.5 subline

XBB.1.5 is a subline of XBB, with an additional spike RBD mutation S486P. This lineage was first detected in the United States with sample collection dates of October 22, 2022, and since then the lineage has grown. As of January 3, 2023, 3,456 sequences have been deposited in GISAID EpiCoV belonging to XBB.1.5, with the mutational profile in the Spike region – Q183E, F486P and F490S. Most of these submissions come from the United States (3,080 sequences) and the United Kingdom (106 sequences). The variant has also been detected in several other countries, including EU/EEA countries – Denmark, France, Austria, Netherlands, Germany, Italy, Spain, Sweden, Iceland, Belgium, Czechia, Portugal and Ireland.

The lineage is currently estimated to have a large growth advantage over previously circulating lines in North America (139%) and Europe (137%), although these estimates are associated with significant uncertainty. The United States Centers for Disease Prevention and Control (US CDC) reports a doubling time of nine days in the proportion of XBB.1.5 and the US-CDC Nowcasting System estimates the current proportion of the variant at around 40% in the United States, with the variant likely to become dominant in the country within a week. The US CDC also presented growth data comparing XBB.1.5 to previously successful variants, indicating that XBB 1.5 has the second highest growth advantage to date, second only to BA.1 (the original Omicron line) . This does not necessarily mean that the variant will become dominant in the EU/EEA, since major differences in the circulation of variants have been observed between North America and Europe on several occasions during the pandemic.

The most likely explanation for the growth advantage is the already high level of immune evasion demonstrated by XBB, combined with the effect of the S486P spike change, which could provide either a transmissibility advantage or additional immune evasion. , or both. This mutation was previously rare during the pandemic, likely because it required two amino acid substitutions in the same codon to change from phenylalanine to proline. In fact, other variants with this change have sprung up before without success. Further laboratory and epidemiological investigations are needed to elucidate the mechanism of the growth advantage conferred by this change for the XBB variant in particular. There is currently not enough information available to assess any change in the severity of infection associated with the variant.

This variant may have an increasing effect on the number of COVID-19 cases in the EU/EEA, but not in the coming month, as the variant is currently only present in the EU/EEA. at very low levels.