Credit NIAID
#17,215
Up until a week ago, Omicron XBB.1.5 was apparently flying beneath the CDC's radar - and reported only as part of its parental XBB subvariant - which was estimated at 18% of cases in the United States 2 weeks ago.
Last week, the CDC's Nowcast broke XBB.1.5 out as a separate entity for the first time, debuting at a roughly 40% share of all cases in the country. We should get our second progress report from the CDC on XBB.1.5 later today.
Over the past week we've seen several attempts to quantify the threat posed by XBB.1.5 (see PrePrint: Enhanced Transmissibility of XBB.1.5 is Contributed by Both Strong ACE2 Binding and Antibody Evasion), but - other than its obviously enhanced transmissibility - most of what we know is based on the older XBB subvariant.
We are seeing increased rates of COVID hospitalization in some parts of the country where XBB.1.5 is strongest, but it is too early to say whether XBB/XBB.1.5 produces any greater severity of illness than earlier Omicron variants.
What is clear is that XBB.1.5 is on track to dominate the rest of the COVID field - at least here in the United States - for the immediate future. And based on its rapid growth - and it already turning up in Europe and Asia - it seems likely to embark on an expanded world tour in the weeks ahead.
Yesterday the ECDC published a brief assessment on XBB.1.5 (see below), but cautions that it is far from a foregone conclusion that XBB.1.5 will have the same degree of traction in Europe.
First their assessment, then I'll return with a brief postscript.
News5 Jan 2023
XBB.1.5 is a sub-lineage of the SARS-CoV-2 lineage XBB, and is currently estimated to have a large growth advantage over previously circulating lineages in North America (139%) and Europe (137%), although these estimates are associated with significant uncertainty. There is a possibility that this variant could have an increasing effect on the number of COVID-19 cases in the EU/EEA, but not within the coming month as the variant is currently only present in the EU/EEA at very low levels.
XBB.1.5 is a sub-lineage of XBB, with an additional spike RBD mutation S486P. This lineage was first detected in the US with sample collection dates as of 22 October 2022, and since then the lineage has been increasing. As of 3 January 2023, 3 456 sequences had been deposited in GISAID EpiCoV belonging to XBB.1.5, with the mutational profile in Spike region - Q183E, F486P and F490S. Most of these submissions are from the US (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 lineages in North America (139%) and Europe (137%), although these estimates are associated with significant uncertainty. The US Centers for Disease Prevention and Control (US CDC) report a doubling time of nine days in the proportion of XBB.1.5 and the US-CDC nowcast system estimates the current proportion of the variant to be around 40% in the US, 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 exhibits the second highest growth advantage to date, second only to BA.1 (the original Omicron lineage). This does not necessarily mean that the variant will become dominant in the EU/EEA, since major differences in variant circulation have been observed between North America and Europe several times during the pandemic.
The most likely explanation of the growth advantage is the already high level of immune escape demonstrated by XBB, combined with the effect of the spike change S486P, which could provide either a transmissibility advantage, additional immune escape, or both. This mutation has previously been rare during the pandemic, probably due to it requiring two amino acid substitutions in the same codon to change from phenylalanine to proline. In fact, other variants with this change have emerged before without becoming successful. Further laboratory and epidemiological investigations are required to elucidate the mechanism of the growth advantage conferred by this change for the XBB variant specifically. There is currently not enough information available to assess any change in infection severity associated with the variant.
There is a possibility that this variant could have an increasing effect on the number of COVID-19 cases in the EU/EEA, but not within the coming month as the variant is currently only present in the EU/EEA at very low levels.
Two months ago, the CDC announced that Omicron BA.5 - which had been the dominant COVID variant since last summer - Dropped Below 40% Of Cases Nationwide, as the tag-team of BQ.1/BQ.1.1 surged ahead.
Their reign was short-lived (< 2 months), as XBB/XBB.1.5 overtook them as of last week.
The first major COVID variant (D614G) lasted nearly a year, before giving way to Alpha in late 2020. While there were other regional contenders (Beta, P1, Gamma, etc.) Alpha dominated for about 6 months, only to be supplanted by Delta, which also reigned for about 6 months. Since then, we've been on a roller coaster of new Omicron variants with each lasting 2 to 4 months.
A month ago, few would have suggested that BQ.1/BQ.1.1 were already on their way out the door, or that XBB would be the heir apparent.
With Omicron, the status quo continues to be very short-lived, making what happens with the virus in 2023 increasingly unpredictable.
Stay tuned.
