WHO, What we know and are still trying to learn about the Omicron variant

A virus you don’t want to get. WHO photo.

Update on Omicron

by the World Health Organization

On 26 November 2021, WHO designated the variant B.1.1.529 a variant of concern (VOC), on the basis of advice from WHO’s Technical Advisory Group on Virus Evolution. The variant has been given the name Omicron. Omicron variant is a highly divergent variant with a high number of mutations, including 26-32 in the spike protein, some of which are concerning and may be associated with immune escape potential and higher transmissibility. However, there are still considerable uncertainties. As of 9 December 2021, cases of human infections with this variant have been identified in 63 countries across all six WHO regions. Current understanding of the Omicron variant from recent data are likely to evolve as more data becomes available.

The overall threat posed by Omicron largely depends on three key questions, including: (1) how transmissible the variant is; (2) how well vaccines and prior infection protect against infection, transmission, clinical disease and death; and (3) how virulent the variant is compared to other variants. Public health advice is based on current information and will be tailored as more evidence emerges around those key questions.

Based on current limited evidence Omicron appears to have a growth advantage over Delta. It is spreading faster than the Delta variant in South Africa where Delta circulation was low, but also appears to spread more quickly than the Delta variant in other countries where the incidence of Delta is high, such as in the United Kingdom. Whether Omicron’s observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain. However, given the current available data, it is likely that Omicron will outpace the Delta variant where community transmission occurs.

There are still limited data on the clinical severity of Omicron. While preliminary findings from South Africa suggest it may be less severe than Delta, and all cases reported in the EU/EEA to date have been mild or asymptomatic, it remains unclear to what extent Omicron may be inherently less virulent. More data are needed to understand the severity profile.

There are limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for Omicron. Preliminary evidence, and the considerably altered antigenic profile of the Omicron spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with Omicron. There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion. In addition, preliminary evidence from a few studies of limited sample size have shown that sera obtained from vaccinated and previously infected individuals had lower neutralization activity (the size of the reduction ranges considerably) than with any other circulating VOCs of SARS-CoV-2 and the ancestral strain.

The diagnostic accuracy of routinely used PCR and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be influenced by Omicron. Most Omicron variant sequences reported include a deletion in the S gene, causing some S gene targeting PCR assays to appear negative. Although some publicly shared sequences lack this deletion, this remains a minority of currently available sequences, and S gene target failure (SGTF) can therefore be used as a useful proxy marker of Omicron, for surveillance purposes. However, confirmation should be obtained by sequencing, as this deletion can also be found in other VOCs (e.g., Alpha and subsets of Gamma and Delta).

Therapeutic interventions for the management of patients with severe or critical COVID-19 associated with the Omicron variant that target host responses (such as corticosteroids, and interleukin 6 receptor blockers and prophylaxis with anticoagulation) are expected to remain effective. However, monoclonal antibodies will need to be tested individually, for their antigen binding and virus neutralization and these studies should be prioritized.


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