Omicron Variant

On 26 November, WHO designated SARS-CoV-2 variant B.1.1.529 a variant of concern (VOC) and named it Omicron¹. Omicron has many key mutations compared to Delta and is now present in almost every country including Armenia.

• Omicron shows 
➢ significant increase in growth rate; 
➢ increased risk of a close contact becoming a secondary case; and 
➢ increase in observed number of people infected by index case compared to Delta 
• High growth rate likely due to a combination of factors including: 
➢ immune evasion (virus evades the protective immune system) and 
➢ potential intrinsic increased transmissibility 
• Omicron has a clear growth advantage over Delta and is rapidly replacing other variants that are circulating in countries

• Omicron appears to show preference for infecting and replicating in the upper respiratory tract, compared to Delta and other strains which prefer the lower respiratory tract. 
• This may confer a transmission advantage independent of immune evasion. 
• Preliminary studies suggest that Omicron appears to have decreased ability to infect lung tissue, which may be a reason why people infected with Omicron have a less severe disease compared to Delta. 
• Early studies from animal models show that Omicron-infected animals show fewer clinical signs and have less severe disease.

• Omicron has reduced risk of hospitalization compared to Delta, suggest early studies from several countries including Denmark, South Africa, UK, Canada and the USA 
• There is decoupling between case reports and hospitalization in places of high levels of population immunity 
• Omicron infection appears to be associated with lower severity and lower proportion of hospitalized patients compared to previous variants, but the large number of people being infected with it translates into significant number of patients requiring hospital admission, putting strain on healthcare systems.

• Routinely used diagnostic tests, including PCR and antigen detection rapid diagnostic tests (Ag-RDT), continue to detect infection with Omicron 
• Studies of the comparative sensitivity of Ag-RDTs are ongoing

• There is evidence that Omicron has some immune evasion 
• Increased trend in re-infection cases have been reported by some countries including South Africa, UK, Denmark, Israel 
• The risk of reinfection with the Omicron variant was estimated to be 5.4 fold higher in comparison with the Delta variant in England, show UK studies 
• Significant reduction in antibody neutralization with Omicron may contribute to increased risk of re-infection.

• Protection through cellular immunity appears to be preserved in Omicron infection 
• In those who have been previously infected, and/or previously vaccinated, 70-80% of certain cells involved in the protective immune process were maintained for Omicron infection 
• This likely translates to protection against severe disease and death after vaccination and after previous infection, remaining high.

• There are many studies underway that are looking at the effectiveness of COVID-19 vaccines and Omicron 
• All initial vaccine effectiveness estimates show reduced effectiveness against infection and symptomatic disease than for Delta; However, estimates of protection remain high for severe end of disease spectrum 
• This means that current COVID-19 vaccines are providing strong protection against severe disease and death if infected with all variants circulating, including Omicron 
• Preliminary vaccine effectiveness estimates appear greater following booster than primary series for most products; no data on inactivated vaccines 
• Unvaccinated cases were more likely to infect household members than those vaccinated or with previous infection. Booster doses further lowered of risk for household members.

Older people and those with underlying conditions remain at risk

¹ WHO. Update 72 – SARS-CoV-2 variant of concern Omicron