SARS-CoV-2 variant of concern Omicron
- • Omicron is highly transmissible and is rapidly replacing Delta as the dominant SARS-CoV-2 variant
• Omicron appears to show preference for infecting the upper respiratory tract, unlike other SARSCoV-2 variants of concern
• There is increasing evidence of immune evasion as Omicron shows increased risk of both reinfection and breakthrough infection after vaccination
• Vaccines protect against hospitalization but are less effective against Omicron symptomatic disease; and booster doses increase vaccine effectiveness
• In places with high population immunity, Omicron also appears to have a reduced risk of severe disease and hospitalizations
• Higher incidence of cases and milder infection with Omicron has led to a decoupling of cases and hospitalization rates, but there are significant numbers of hospitalized patients as a result of the high levels of transmission
• Older persons and those with underlying conditions continue to be at high risk of severe disease
• Measures such as wearing a well-fitting mask properly, keeping physical distance and other public health and social measures continue to protect against all SARS-CoV-2 variants.
Omicron is highly transmissible
• 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 shows preference for upper respiratory tract infection
• 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
• 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.
Routine diagnostics continue to detect Omicron
• 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
Omicron shows increased re-infection risk after previous SARS-CoV-2 infection
• 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.
Preserved cellular immune process protects against severe disease
• 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.
Vaccines protect against hospitalization but are less effective against Omicron symptomatic disease
• 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
• Those with underlying conditions, of any age, are also at risk for developing severe disease.