Overview 

• On 26 November 2021, the World Health Organization ( W.H.O.) 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.

WHO TEAM

WHO Headquarters (HQ),  WHO Health Emergencies Programme

EDITORS

World Health Emergencies Programme

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