Enhancing Readiness for Omicron (B.1.1.529): Technical Brief and Priority Actions for Member States
- 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 humoral immune escape potential and higher transmissibility.
- As of 16 December 2021, the Omicron variant has been identified in 89 countries across all six WHO regions. Current understanding of the Omicron variant will continue to evolve as more data becomes available.
- The overall threat posed by Omicron largely depends on four key questions, including: (1) how transmissible the variant is; (2) how well vaccines and prior infection protect against infection, transmission, clinical disease and death; (3) how virulent the variant is compared to other variants; and (4) how populations understand these dynamics, perceive risk and follow control measures, including public health and social measures. Public health advice is based on current information and will be tailored as more evidence emerges around those key questions.
- There is consistent evidence that Omicron has a substantial growth advantage over Delta. It is spreading significantly faster than the Delta variant in countries with documented community transmission, with a doubling time between 1.5–3 days. Omicron is spreading rapidly in countries with high levels of population immunity and it remains uncertain to what extent the observed rapid growth rate can be attributed to immune evasion, intrinsic increased transmissibility or a combination of both. However, given current available data, it is likely that Omicron will outpace Delta where community transmission occurs.
- There are still limited data on the clinical severity of Omicron. More data are needed to understand the severity profile and how severity is impacted by vaccination and pre-existing immunity. Hospitalizations in the UK and South Africa continue to rise, and given rapidly increasing case counts, it is possible that many healthcare systems may become quickly overwhelmed.
- Preliminary data suggest that there is a reduction in neutralizing titres against Omicron in those who have received a primary vaccination series or in those who have had prior SARS-CoV-2 infection , which may suggest a level of humoral immune evasion.
- There are still limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for Omicron. Preliminary findings of vaccine effectiveness studies (test-negative design) were obtained from South Africa and England, the United Kingdom. Available preliminary data to be interpreted with caution as the designs may be subject to selection bias and the results are based on relatively small numbers. Results from England indicate a significant reduction in vaccine effectiveness against symptomatic disease for Omicron compared to Delta after two vaccine doses of either Pfizer BioNTech-Comirnaty or AstraZeneca-Vaxzevria vaccines. There was, however, higher effectiveness two weeks after a Pfizer BioNTech-Comirnaty booster, which was slightly lower or comparable to that against Delta. A non peer-reviewed study by South Africa researchers using private health insurance data reported reductions in vaccine effectiveness of the Pfizer BioNTech-Comirnaty vaccine against infection, and to a lesser degree against hospitalization. Details about the methods or results were not available at the time of writing.
- The diagnostic accuracy of routinely used PCR and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be impacted by Omicron. Most Omicron variant sequences reported include a deletion in the S gene, which can cause an S gene target failure (SGTF) in some PCR assays. Though a minority of publicly-shared sequences lack this deletion, SGTF can be used as a proxy marker to screen for Omicron. 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) circulating at low frequencies globally.
- 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) are expected to remain effective. However, preliminary data from preprint publications suggest that some of the monoclonal antibodies developed against SARS-CoV-2 may have decreased neutralization against Omicron. 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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