SARS-CoV-2 variants of concern and variants under investigation in England
Technical briefing 33
23 December 2021
This briefing provides an update on previous briefings up to 17 December 2021
Severity
The risk of hospital admission for a person detected as a case of Omicron appears reduced compared to a case of Delta. This analysis excludes known reinfections. The current hazard ratio is 0.62 (95%CI 0.55-0.69) for emergency department attendance or admission, and 0.38 (95% CI 0.3-0.5) for admission alone. This analysis is preliminary because of the small numbers of Omicron cases currently in hospital and the limited spread of Omicron into older age groups as yet. It has not been adjusted for undiagnosed reinfections. It will be iterated regularly. In addition, Imperial reported analysis using the same data set but imputing a potential previous infection variable and estimated the intrinsic risk difference between Delta and Omicron as between 0 to 30% and the reduced risk of hospitalisation in those previously infected estimated as 55 to 70%. In the Scottish study, the range of estimates for their analysis was similar, though based on only 18 total admissions detected for Omicron in the study and only 7 individuals admitted with 7 or more days of follow-up.
Vaccine effectiveness
Repeated VE analysis continues to show lower VE for symptomatic Omicron disease compared to Delta. There is evidence of waning of protection against symptomatic disease with increasing time after dose 2, and by 10 weeks after the booster dose, with a 15 to 25% reduction in vaccine effectiveness after 10 weeks. This waning is faster for Omicron than for Delta infections. There are insufficient severe cases of Omicron as yet to analyse vaccine effectiveness against hospitalisation, but this is expected to be better sustained, for both primary and booster doses. This analysis will be iterated next week, although numbers may still restrict a robust analysis of protection against more severe outcomes. The VE data will also appear in the weekly COVID-19 vaccine surveillance report published routinely on a Thursday.
Reinfections
The population reinfection rate has increased sharply and disproportionately to the number of first infections. 9.5% of Omicron infections have been identified to have previous confirmed infections, which is likely to be a substantial underestimate of the proportion of reinfections. The first infections of the individuals with Omicron reinfections occurred in both the Alpha and Delta waves and are likely to have been undetected if in the first wave. There were 69 identified cases with Omicron as a third episode of infection and 290 cases where the Omicron infection was between a 60 to 89 day interval after a confirmed first infection.
A total of 14 people have been reported to have died within 28 days of an Omicron COVID-19 diagnosis. The median time from Omicron specimen date to death was 4 days (range 1 to 10). The age of those dying ranged from 52 to 96 years.
Stratified Cox proportional hazard regression assessed that the risk of presentation to emergency care or hospital admission with Omicron was approximately three-fifths of that for Delta (Hazard Ratio 0.62, 95% CI: 0.55 to 0.69). The risk of hospital admission alone with Omicron was approximately two-fifths of that for Delta (Hazard Ratio 0.38, 95% CI: 0.30 to 0.50). These analyses stratified on week of specimen and area of residence and further adjusted for age, exact calendar date, sex, ethnicity, local area deprivation, international travel and vaccination status.
This effect is still present when stratified by vaccination status. However, this is preliminary analysis including only 431 attendances to the emergency department and 70 hospital admissions with Omicron. These analyses also are not adjusted for undiagnosed previous COVID-19 infection, or co-morbidities of these individuals. It is not an assessment of in hospital severity, which will take further time to access. Despite adjusting for calendar week, there may still be reporting delays for hospital events. It is important to highlight that these lower risks do not necessarily imply reduced hospital burden over the epidemic wave given the higher growth rate and immune evasion observed with Omicron.
) that on the community side the cause is lax personal discipline and carelessness. Despite the success of hard lockout/lockdown the overall numbers have been consistently low. Most people have no real appreciation of infection as a risk distinct from the frustrations of "obeying the rules". (Yes I'm not in a capital city but I have so far not had a known single case of infection in my range of contacts.)
