New Delhi: The World Health Organization (WHO) on Thursday said the Alpha, Beta, Gamma, Delta and Omicron parent lineage (B.1.1.529) of Sars-CoV-2 are considered “previously circulating variants of concern (VOCs)”, while XBB.1.5 is a variant of interest (VOI), as it updated its tracking system and working definitions to better correspond to the current global variant landscape.
The main update consists in making the VOC definition more specific, to include major Sars-CoV-2 evolutionary steps that require major public health interventions. In addition, going forward, WHO will assign Greek labels for VOCs, and will no longer do so for VOIs.
It emphasised that the changes to the categorisation of the variants do not imply that the circulation of Omicron viruses no longer pose a threat to public health. Rather, the changes have been made in order to better identify additional or new threats over and above those posed by the current Omicron viruses in circulation.
Since its emergence, Omicron viruses have continued to evolve genetically and antigenically with an expanding range of sub-lineages, which so far have all been characterised by properties of evasion of existing population immunity and a preference to infect the upper respiratory tract (versus lower respiratory tract), as compared to pre-Omicron VOCs.
The Omicron viruses account for over 98% of the publicly available sequences since February 2022 and constitute the genetic background from which new Sars-CoV-2 variants will likely emerge, although the emergence of variants derived from previously circulating VOCs or of completely new variants remains possible.
“Based on comparisons of antigenic cross reactivity using animal sera, replication studies in experimental models of the human respiratory tract, and evidence from clinical and epidemiological studies in humans, there is consensus among experts in WHO’s Technical Advisory Group on Sars-CoV-2 Virus Evolution (TAG-VE) that compared to previous variants, Omicron represents the most divergent VOC seen to date,” the WHO said in a statement.
“The previous system classified all Omicron sub-lineages as part of the Omicron VOC and thus did not have the granularity needed to compare new descendent lineages with altered phenotypes to the Omicron parent lineages (BA.1, BA.2, BA.4/BA.5). Therefore, from March 15, 2023, the WHO variant tracking system will consider the classification of Omicron sub-lineages independently as variants under monitoring (VUMs), VOIs, or VOCs,” it said.
Since the beginning of the Covid-19 pandemic, multiple VOCs and VOIs have been designated by the UN health agency based on their assessed potential for expansion and replacement of prior variants, causing new waves with increased circulation, and the need for adjustments to public health actions.
WHO will also continue to issue regular risk assessments for both VOIs and VOCs.