The World Health Organisation, on Thursday, updated its tracking system and working definitions for variants of SARS-CoV-2, the virus that causes COVID-19.
WHO in a statement said that the tracking system would better correspond to the current global variant landscape, to independently evaluate Omicron sublineages in circulation, and classify new variants more clearly when required.
It said: “SARS-CoV-2 continues to evolve. Since the beginning of the COVID-19 pandemic, multiple Variants of Concern (VOCs) and Variants of Interest (VOIs) have been designated by WHO.
“This is based on their assessed potential for expansion and replacement of prior variants, for causing new waves with increased circulation, and for the need for adjustments to public health actions.
“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.’’
WHO said since its emergence, Omicron viruses had continued to evolve genetically and antigenically with an expanding range of sublineages, a preference to infect the upper respiratory tract (versus lower respiratory tract), as compared to pre-Omicron VOCs.
According to it, Omicron viruses account for more than 98 per cent of the publicly available sequences since February 2022 and constitute the genetic background from which new SARS-CoV-2 variants will likely emerge.
It said that the emergence of variants derived from previously circulating VOCs or of completely new variants remained possible.
WHO said the previous system classified all Omicron sublineages as part of the Omicron VOC and thus does not have the granularity needed to compare new descendant lineages with altered phenotypes to the Omicron parent lineages (BA.1, BA.2, BA.4/BA.5).
“Therefore, from March 15, the WHO variant tracking system will consider the classification of Omicron sublineages independently as variants under monitoring (VUMs), VOIs, or VOCs.
“WHO is also updating the working definitions for VOCs and VOIs.
“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.
“For the updated definitions, please visit the WHO variant tracking website,” it said.
According to the organisation, going forward, WHO will assign Greek labels for VOCs, and will no longer do so for VOIs.
It said that with these changes factored in, Alpha, Beta, Gamma, Delta as well as the Omicron parent lineage (B.1.1.529) were considered previously circulating VOCs, and were now classified XBB.1.5 as a VOI.
It said that WHO would also continue to issue regular risk assessments for both VOIs and VOCs (see latest risk assessment for XBB.1.5).
It said that WHO stressed that these changes did not imply that the circulation of Omicron viruses no longer posed a threat to public health.
WHO said that rather, the changes have been made to better identify additional or new threats over and above those posed by the current Omicron viruses in circulation.