Omicron SARS-CoV-2 variant: What we know so far

There is increasing concern about the omicron variant. This article will summarize what we know about the variant so far. Our knowledge about the omicron variant is evolving so there may be changes in our understanding later.

Background Information:

The technical name for the omicron variant is B.1.1.529 and it was designated a variant of concern by the World Health Organization (WHO) on 26 November 2021.

Omicron has a high number of mutations (26-32 mutations in the spike protein), some of which are likely to be associated with higher transmissibility and humoral immune escape potential.

Key Messages:

The overall risk related to Omicron remains very high as

  • the global risk of COVID-19 remains very high
  • current data indicate that Omicron tends to spread more rapidly than Delta. The rapid increase in cases will lead to increased hospitalizations, lead to significant morbidity in vulnerable populations, and may pose overwhelming demands on health care systems.

The overall threat posed by Omicron largely depends on four key questions:

  • how transmissible the variant is;
  • how well vaccines and prior infection protect against infection, transmission, clinical disease and death;
  • how virulent the variant is compared to other variants; and
  • how populations understand these dynamics, perceive risk and follow control measures, including public health and social measures (PHSM).

Summary of current evidence:

Impact on epidemiology

The Omicron variant has been identified in 149 countries across all six WHO Regions.

There is consistent evidence that Omicron has a substantial growth advantage over Delta. The Omicron variant is spreading significantly faster than the Delta variant in countries with documented community transmission.

Early data from South Africa, the United Kingdom, Canada and Denmark suggest a reduced risk of hospitalization for Omicron compared to Delta. More data across different countries are needed to understand how clinical markers of severity– such as the use of oxygen, mechanical ventilation and deaths– are associated with Omicron. Early data suggests that, as with all other variants of SARS-CoV-2, severity of Omicron increases with age and in the presence of underlying medical conditions, as well as among people who are not vaccinated. Moreover, current evidence about severity and hospitalization comes largely from countries with high levels of population immunity, and there remains uncertainty about the severity of Omicron in populations with different vaccination coverage and prior exposure to other variants.

Note: Despite the lower risk of hospitalization compared to Delta, the higher incidence of Omicron will lead to a substantial increase in hospitalizations overall.

Impact on diagnostics and testing

The diagnostic accuracy of routinely used PCR and antigen-detection rapid diagnostic tests (Ag-RDT) assays does not appear to be impacted by Omicron; studies of the comparative sensitivity of Ag-RDTs are ongoing.

Impact on immunity (following infection or vaccination)

Preliminary data from multiple non-peer reviewed studies suggest that there is a reduction in neutralizing titers against Omicron in individuals who have received a primary vaccination series or in those who have had prior SARS-CoV-2 infection. In addition, increased risk of reinfection has been reported in South Africa, the United Kingdom, Denmark, and Israel, all suggesting immune evasion against Omicron.

Early data suggests that the effectiveness of studied vaccines is significantly lower against Omicron infection and symptomatic disease compared to Delta, with homologous (receiving a booster dose of the same vaccine used for primary vaccination) and heterologous (receiving a booster dose of a different vaccine than that used for primary vaccination) booster doses increasing vaccine effectiveness. There is one study showing decreasing effectiveness of the booster dose against symptomatic disease over time. More data are needed to assess this preliminary finding across studies, vaccine platforms and dosing regimens. There are no effectiveness data for several vaccines, particularly the inactivated vaccines.

Impact on host tropism, virus fitness and pathogenicity

Preliminary evidence suggests a potential shift in tropism of the Omicron variant towards the upper respiratory tract, as compared to Delta and the wild type (WT) virus that have a tropism for the lower respiratory tract.

Impact on therapeutics and treatments

Therapeutic interventions for the management of patients with severe or critical Omicron-associated COVID-19 that target host responses (such as corticosteroids, and interleukin-6 receptor blockers) are expected to remain effective. However, preliminary data from non-peer reviewed publications suggest that some of the monoclonal antibodies developed against SARS-CoV-2 may have impaired neutralization against Omicron. Monoclonal antibodies will need to be tested individually for their antigen binding and virus neutralization, and these studies should be prioritized.

Link to related World Health Organization (WHO) technical document:

Link to World Health Organization (WHO) COVID-19 page:


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