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
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Comparative demographics
Relative to Delta, Omicron is currently more concentrated in young adult age groups (20
to 29) and is less prevalent in children. Whilst there were initially higher rates of Omicron
cases in persons of Black ethnicity, the rates of all ethnic groups have now converged
reflecting widespread community transmission. These demographic factors should be
borne in mind when interpreting comparative analyses.
Hospitalisation and death
Using data up until 20 December, 132 individuals with laboratory confirmed Omicron
have been admitted or transferred from emergency departments. Over 40% of
admissions were in London.
Of those patients admitted to hospital, 17 (12.9%) had
received a booster dose, 74 (56.1%) a second dose and 27 (20.5%) were not vaccinated
(less than 10 were unlinked or had one dose). At the data cut off, 14 people were
reported to have died within 28 days of an Omicron diagnosis, age range 52 to 96 years.
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.
Secondary attack rates
Iterated secondary attack rates calculated using routine contact tracing continue to show
higher secondary attack rates for Omicron than for Delta.
The difference between
Omicron and Delta is currently greater for non-household contacts than for household
contacts
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2.3 Severity
Descriptive epidemiology of severe outcomes of Omicron in England
To monitor the severe outcomes of Omicron infections, Omicron cases are linked to NHS data
on presentation to emergency care and to UKHSA data on deaths following confirmed COVID19 test results.
Hospitalisation was defined as attendance to emergency care which resulted in
admission or transfer, and the Omicron specimen date was between 14 days prior to
attendance and 1 day after attendance.
Using data up until 20 December 2021, a total of 132 individuals with laboratory-confirmed
(sequencing, genotyping or SGTF) Omicron have been admitted or transferred from emergency
departments. Of these, 54 (40.9%) admissions were in London.
The age range of admitted individuals was 0 to 98, years (median: 45.5); 74 (56.1%) were aged
40 years or more; 25.8% were aged 70 years or more. 55% of Omicron hospitalisations
occurred in people whose self-reported ethnicity was White (British) and 8% among Black
(African) people.
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.
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2.4 Vaccine effectiveness
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Vaccine effectiveness was estimated by period after dose 2 and dose 3. The final analysis
included 147,597 Delta and 68,489 Omicron cases. Vaccine effectiveness against symptomatic
disease by period after dose 2 and dose 3 is shown in Figure 7 for those who received a
primary course of the AstraZeneca vaccine (Figure 10A), Pfizer (Figure 10B) or Moderna
(Figure 10C). Booster estimates are separated for Pfizer and Moderna boosters. In all periods,
effectiveness was lower for Omicron compared to Delta.
Among those who received an
AstraZeneca primary course, vaccine effectiveness was around 60% 2 to 4 weeks after either a
Pfizer or Moderna booster, then dropped to 35% with a Pfizer booster and 45% with a Moderna
booster by 10 weeks after the booster. Among those who received a Pfizer primary course,
vaccine effectiveness was around 70% after a Pfizer booster, dropping to 45% after 10-plus
weeks and stayed around 70 to 75% after a Moderna booster up to 9 weeks after booster.
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