The SIMID consortium prepared a new technical note (v2022-01-05) containing the model estimates of hospital admissions and load by stochastic dynamic transmission model using observational data up to Janary 3rd, 2022.
We explored the impact of the Omicron VOC for Belgium with a country-specific stochastic transmission model that incorporates infection- and vaccine-induced immunity levels in the population given the ongoing COVID-19 pandemic. By combining estimates on the predicted penetration of Omicron based on S-gene-target failure data for Belgium with current estimated transmission dynamics in Belgium, the model projects increasing numbers of infections and hospitalisations by the Omicron VOC in the coming weeks, with hospital admissions likely exceeding those observed in the fourth wave.
Although the relative risk of hospital admission per infection is estimated from international observations to be lower for Omicron than for Delta, the expected high incidence of infections penetrating all age groups, could still cause a large burden on the healthcare system, both in primary care and in hospitals.
We estimated the increase in transmissibility of Omicron relative to Delta based on S-gene-target failure (SGTF) data from Belgium between 30% and 80%, depending on the assumptions for vaccine-related protection against Omicron. In addition, the latest model calibration resulted in a much faster transition to the pre-symptomatic infectious stage after infection with Omicron, relative to Delta. This aligns with a shorter serial interval for Omicron, as reported by Kim et al (2021, preprint), hence the transmission advantage of Omicron in our projections is not only based on immune escape and increased infectiousness.
In this technical note it is particularly difficult to properly calibrate the model and define scenarios for the following reasons: (1) The Omicron VOC has only become completely dominant near the end of the Christmas holiday period and the observed hospital admissions (to which the model is calibrated) during that period were a result of (mostly) Delta infections and (to a much lesser extent) Omicron infections acquired before and at the start of the holiday period; (2) social contact behaviour is known to be very different during holiday versus non-holiday periods, not only in terms of numbers, but also in terms of age-specificity, and we are currently only starting to observe the impact of infections that occurred during the Christmas holidays on hospital admissions (to which the model is calibrated, note also that recent data on serology in unvaccinated persons are not available as an additional source for calibration); (3) We currently have no observations on hospital admissions caused by infections acquired in a non-holiday period (with associated non-holiday contact behaviour) while Omicron is the dominant VOC in Belgium, which would allow us to establish the relationship between age-specific contacts, infections and hospital admissions that is representative of the period for which projections are made in this technical note; (4) policy makers made important changes to the rules of quarantine and isolation from 10th January onwards, which is likely to alter the transmission dynamics in comparison to the previous months, over and above the expected changes instigated by the full dominance of Omicron in Belgium.
The SIMID consortium wrote also a technical note in December 2021 to share with the Superior Health Council on COVID-19 vaccine uptake for 5-11-year old children. The full Technical Note from December (v2021-12-09) is available here.