The SIMID consortium prepared a new technical note (v2022-04-15) containing the estimates of a stochastic dynamic transmission model using observational data up to April 11th, 2022.

Conclusions

• We explored the impact of the Omicron Variant of Concern (VOC) for Belgium with a country-level stochastic transmission model that incorporates infection- and vaccine-induced immunity levels in the population. Under a baseline scenario without any future change in currently estimated transmission dynamics or circulating VOCs, the model projects decreasing numbers of infections and hospital admissions and load in the coming weeks. This trend is caused by a persisting decrease in overall susceptibility in the population, even when we account for waning of vaccine-induced and natural immunity over time. Due to uncertainty with regard to the dynamics for March-April 2022, we explored different assumptions on drivers for these dynamics on the projected outcomes in terms of hospital admissions and load for May-June 2022.

• We explored the impact of additional booster doses by increasing the age-specific booster uptake in the population to the uptake level of at least two doses of a COVID-19 vaccine by May 15th, 2022. With increased transmission dynamics for March-April 2022, the model output shows only small differences between the scenarios on booster dose uptake. The incremental effect of the additional booster doses decreases when the circulation of the virus decreases. Note that the model does not account for local differences in immunity and clustered social contact networks. General trends are captured well, though local outbreaks are underestimated and herd immunity effects are overestimated in sub-populations with immunity levels below the national level.

• We explored a counterfactual historical scenario with an overall 50% reduction in booster dose uptake in the past, while all other factors (i.e., social contact behaviour, VOC, etc.) are assumed to remain constant based on the most up to date available information from the literature, empirical observations and model calibration. As expected, the model shows that the peak in hospital admissions and load in January 2022 could have been much higher (as a direct result of lower vaccine-induced immunity levels) while the peak of the second Omicron wave in March could have been lower. The model accounts for substantial waning of immunity against infection with the Omicron VOC after two doses of any vaccine type, though protection against severe disease is considered to remain substantial. The booster dose re-establishes protection against infection and severe disease, which explains the lower hospital load in the scenario based on the reported uptake. A higher number of infections in the first year 2022 (mainly Omicron) wave, hence increased natural immunity levels upon recovery, explains the reduced peak of the second wave in 2022. These conclusions hold in the absence of re-infections with the same VOC.

• We are making the implicit assumption that the current Omicron VOC will remain dominant throughout the entire simulation period. Nonetheless, other (new emerging) VOCs may have different transmission probabilities and probabilities to cause disease, hospitalization, death, and different vaccine effectiveness characteristics against each of these manifestations.

The full document from April 2022 (v2022-04-15) is available here.