#17,232
A week ago we looked at a brief 1-page ECDC Assessment of the XBB.1.5 Sub-lineage, where they reported only low levels of that variant having been reported in Europe, and in the very short-term, a low risk to the EU.
Yesterday the ECDC released a more detailed (14-page PDF) TAB (Threat Assessment Brief) that details both what we know, and what we don't know, about this emerging variant.
Although Omicron XBB.1.5 has been described as one of the most immune evasive variants to date, and is demonstrating (at least in the United States) impressive growth, we have not seen any clear signals that it produces any greater severity of illness than earlier Omicron variants.
However, with 90% of the world's countries no longer regularly submitting weekly hospitalization data to WHO, and COVID testing, sequencing, and other data collection falling sharply globally over the past year, our ability to see and quantify emerging COVID threats has diminished substantially.
Much like the old joke about seeing a doctor who couldn't cure you - but for a small fee would touch up your X-rays - we are left with a reassuring, but potentially misleading, prognosis.
This global deficit in reporting and surveillance extends far beyond our current pandemic (see Flying Blind In The Viral Storm), leaving us open to being blindsided by the next emerging disease threat.
With the caveat that everything we think we know is based on `limited data', the ECDC's executive summary, and some excerpts from yesterdays threat assessment brief, follow.
Threat Assessment Brief: Implications for the EU/EEA of the spread of the SARS-CoV-2 Omicron XBB.1.5 sub-lineage
Risk assessment
13 Jan 2023
According to the current ECDC assessment, there is moderate probability of XBB.1.5 becoming dominant in the EU/EEA and causing a substantial increase in the number of COVID-19 cases within the next one to two months.
Executive summary
The XBB.1.5 Omicron sub-lineage was first detected in samples in the United States (US) in October 2022. This sub-lineage has been growing in proportion in the US and many countries around the world since then, including in several EU/EEA countries and the UK. XBB.1.5 is a sub-lineage of XBB, which is a recombinant of two earlier lineages of the Omicron variant of concern (VOC), with an additional spike receptor-binding domain (RBD) change S486P. The reported proportion of XBB.1.5 in the EU/EEA has been lower than 2.5% for the last two weeks of 2022.
XBB.1.5 currently exhibits a daily growth advantage of 12% in the US compared to other circulating variants. It is plausible that the difference in growth rate between XBB and XBB.1.5 is mainly explained by a difference in transmissibility. In addition, XBB and XBB.1 demonstrate the most substantial immune escape observed amongst Omicron sub-lineages to date, with significant reductions in the neutralising capacity of serum from vaccinated individuals. Preliminary in-vitro data show the immune escape properties of XBB.1.5 are equivalent to XBB.1.
While there are currently no vaccine effectiveness (VE) estimates for XBB.1.5, the available vaccines still remain effective against severe disease due to previous and current Omicron variants dominant in the EU, even though there is some evidence of waning over time.
There are currently no signals that the infection severity of XBB.1.5 is different than that of previously circulating Omicron sub-lineages. Regarding therapeutics, no specific data exist on the effectiveness of therapeutics such as nirmatrelvir/ritonavir or remdesivir for XBB.1.5, however the variant is expected to be as susceptible to these antivirals as XBB has shown to be.
Mathematical modelling performed by ECDC provides estimates of when XBB.1.5 might become dominant (i.e. causes more than 50% of infections) in the EU/EEA by using a broad range of scenarios with hypothetical values of the growth rate advantage and of the current proportions of XBB.1.5 in the EU/EEA.
Severity
There are currently no signals that the infection severity of XBB.1.5 is different than that of previously circulating Omicron sub-lineages. However, an increase in cases caused by increased transmissibility or immune escape would be expected to lead to an increase in the number of severe cases. In hamsters, the virological characteristics in vivo and intrinsic pathogenicity of XBB is comparable to the BA.2.75 sub-lineage, and lower than Delta VOC based on body weight, pulmonary function, efficacy of viral spread in the respiratory tissues and histopathological assessments reported in a preprint study [25]. The results suggest that XBB is less pathogenic than Delta and comparable with BA.2.75. To our knowledge, no in vitro or in vivo studies have yet been reported for the XBB.1.5 sub-lineage specifically.
While there are currently significant gaps in our understanding of XBB.1.5, I'm sure the virus will be happen to enlighten us over the weeks and months ahead.
