The World Health Organization (WHO) announced that it has updated its tracking system and working definitions for variants of SARS-CoV-2, to better reflect the current global variant landscape, to independently evaluate Omicron sublineages in circulation, and classify new variants more clearly when required.
Omicron variants now accounts for 98% of circulating SARS-CoV-2, and new variants will likely emerge from this genetic background. Based on its latest risk assessment [PDF], South Africa has reported a strong increase in XBB.1.5 from 1% in December 2022, to 10% in January 2023, and 76% as of the latest report from February 2023.
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, WHO experts conclude that, compared to previous variants, Omicron represents the most divergent variant of concern (VOC) seen so far. Since its emergence, Omicron viruses have continued to evolve genetically and antigenically with an expanding range of sublineages, which so far have had all been characterised by 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 previous system classified all Omicron sublineages as part of the Omicron VOC and was not detailed enough to compare new descendent lineages with altered phenotypes to the Omicron parent lineages (BA.1, BA.2, BA.4/BA.5). So, WHO variant tracking system will consider the classification of Omicron sublineages independently as variants under monitoring (VUMs), variants of interest (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.
In addition, going forward, WHO will assign Greek labels for VOCs, and will no longer do so for VOIs.
With these changes factored in, Alpha, Beta, Gamma, Delta as well as the Omicron parent lineage (B.1.1.529) are considered previously circulating VOCs. WHO has now classified XBB.1.5 as a VOI.
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.
