WHO has updated its tracking system and working definitions for variants of SARS-CoV-2, the virus that causes COVID-19, to better match the current global variant landscape, independently assess circulating Omicron sublines and classifying new variants more clearly if necessary.
SARS-CoV-2 continues to evolve. Since the onset of the COVID-19 pandemic, several Variants of Concern (COVs) and Variants of Interest (VOIs) have been designated by WHO based on their assessed potential to expand and replace previous variants, to cause new waves with increased circulation, and for the need to adjust public health actions.
Based on comparisons of antigenic cross-reactivity using animal sera, replication studies in experimental human airway models, and evidence from clinical and epidemiological studies in humans, there is consensus among experts from the WHO Technical Advisory Group on the Evolution of the SARS-CoV-2 Virus (TAG-VE) that compared to previous variants, Omicron represents the most divergent COV observed to date. Since its emergence, Omicron viruses have continued to evolve genetically and antigenically with an increasing range of sublineages, all of which so far have been characterized by properties of evasion of existing population immunity and a preference for infect the upper respiratory tract (vs. lower respiratory tract), vs. pre-Omicron VOCs.
Omicron viruses represent more than 98% of publicly available sequences as of February 2022 and provide the genetic background from which new variants of SARS-CoV-2 will likely emerge, although the emergence of variants derived from previously circulating COVs or completely new variants is still possible. The previous system classified all Omicron sublines as part of the Omicron COV and therefore lacked the granularity to compare new progeny lines with altered phenotypes to the Omicron parent lines (BA.1, BA.2, BA. 4/BA.5). Therefore, from March 15, 2023, the WHO Variant Tracking System will consider the classification of Omicron sublines independently as Variants Under Surveillance (VUM), VOI or VOC.
The WHO is also updating the working definitions of VOCs and VOIs. The main update is to make the definition of VOCs more precise, to include the major evolutionary stages of SARS-CoV-2 that require major public health interventions. For updated definitions, please visit the WHO variant tracking website.
Also, in the future, the WHO will assign Greek labels to VOCs and will no longer do so for VOIs.
With these changes taken into account, Alpha, Beta, Gamma, Delta as well as the parent line Omicron (B.1.1.529) are considered previously circulating VOCs. The WHO has now classified XBB.1.5 as a VOI.
WHO will also continue to publish risk assessments for VOIs and VOCs on a regular basis (see latest risk assessment for XBB.1.5).
The WHO stresses that these changes do not imply that the circulation of Omicron viruses no longer poses a threat to public health. Rather, the changes were made in order to better identify additional or new threats in addition to those posed by the Omicron viruses currently in circulation.