New Delhi: The World Health Organisation (WHO) has updated its tracking system for variants of SARS-CoV-2, the virus that causes COVID-19, to allow better classification of sublineages of the Omicron variant.
As part of the update, from March 15, 2023, the WHO variant tracking system will consider the classification of Omicron sublineages independently as variants under monitoring (VUMs), variants of interest (VOIs) or variants of concern (VOCs). The categorisation of any variant or subvariants as ‘VOC’ indicates the highest level of concern, preceded by classification as VOI and VUM respectively.
After the update, the Alpha, Beta, Gamma, Delta and the Omicron parent lineage (B.1.1.529) will be considered “previously circulating VOCs”. There are no VOCs circulating as of March 15, 2023. One Omicron subvariant, XBB.1.5, is classified as VOI, while five strains have been classified as VOMs.
All viruses change over time, but most changes have little to no impact on the virus’s properties. But some changes may affect the virus’s properties – how easily it spreads, the associated disease severity, or the performance of vaccines or medicines. Since its emergence in November 2021, Omicron has accounted for the highest proportion of disease – mostly because of its ability to evade immunity.
In a statement, the UN agency said that its Technical Advisory Group on SARS-CoV-2 Virus Evolution (TAG-VE) has agreed that compared to previous variants, Omicron “represents the most divergent” VOC seen to date.
“Since its emergence, Omicron viruses have continued to evolve genetically and antigenically with an expanding range of sublineages,” the WHO said. All the sublineages have been characterised by their ability to evade existing population immunity and a “preference to infect the upper respiratory tract (versus lower respiratory tract), as compared to pre-Omicron VOCs,” it said.
The update to the WHO’s working definitions and tracking system of SARS-CoV-2 variants was triggered by the fact that 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 most likely emerge.
The WHO said that the previous system classified all Omicron sublineages 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)”.
The working definitions for VOCs and VOIs have been updated to make the VOC definition more specific. A VOI will now be classified as a VOC if it is determined that it will require major public health interventions.
Additionally, the WHO said it will assign Greek labels – Alpha, Beta, etc. – only for VOCs, and will no longer do so for VOIs.
“WHO emphasizes that these changes 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,” the statement said.
