https://www.imperial.ac.uk/media/imperial-college/medicine/mrc-gida/2021-12-16-COVID19-Report-49.pdf
3. Discussion
The growth rates estimated for Omicron translate into doubling times of under 2.5 days, even allowing for the potentially slowing of growth up to 11th December. These estimates are consistent or even faster than doubling times reported from South Africa (13). Assuming an exponentially distributed generation time of 5.2 days and that R=1 currently for Delta, reproduction number (R) estimates for Omicron are above 3 for the SGTF and genotype analyses, and above 2.5 even for the period 8th-10th December. Shorter assumed generation times will give lower R estimates.
The distribution of Omicron by age, region and ethnicity currently differs markedly from Delta, indicating Omicron transmission is not yet uniformly distributed across the population. However, we note that given its immune evasion, the age distribution of Omicron infection in the coming weeks may continue to differ from that of Delta. London is substantially ahead of other English regions in Omicron frequency.
We find strong evidence of immune evasion, both from natural infection, where the risk of reinfection is 5.41 (95% CI: 4.87-6.00) fold higher for Omicron than for Delta, and from vaccine-induced protection. Our VE estimates largely agree with those from UKHSA’s TNCC study (11) and predictions from predicting VE from neutralising antibody titres (4,14), suggesting very limited remaining protection against symptomatic infection afforded by two doses of AZ, low protection afforded by two doses of Pfizer, but moderate to high (55-80%) protection in people boosted with an mRNA vaccine.
Our estimate of the hazard ratio for reinfection relative to Delta also supports previous analysis of reinfection risk in South Africa (15). Prior to Omicron, the SIREN cohort study of UK healthcare workers estimated that SARS-CoV-2 infection gave 85% protection against reinfection over 6 months (16), or a relative risk of infection of 0.15 compared with those with no prior infection. Our hazard ratio estimate would suggest the relative risk of reinfection has risen to 0.81 [95%CI: 0.73-1.00] (i.e. remaining protection of 19% [95%CI: 0-27%]) against Omicron.
We find no evidence (for both risk of hospitalisation attendance and symptom status) of Omicron having different severity from Delta, though data on hospitalisations are still very limited.
There are several limitations of this analysis. While case numbers are increasing quickly, there are still limits in our ability to examine interactions between the variables considered. The distribution of Omicron differed markedly from Delta across the English population at the time this analysis was conducted, likely due to the population groups in which it was initially seeded, which increases the risks of confounding in analyses. SGTF is an imperfect proxy for Omicron, though SGTF had over 60% specificity for Omicron over the date range analysed in the SGTF analysis (and close to 100% by 10th December). Intensified contact tracing around known Omicron cases may have increased case ascertainment over time, potentially introducing additional biases.
Our analysis reinforces the still emerging but increasingly clear picture that Omicron poses an immediate and substantial threat to public health in England and more widely.
