Figure 1.

Approximation of vaccine impact on the risk of hospitalisation through the estimation of risk ratios (RR) between October 2021 and February 2022, based on the analyses of aggregated hospitalisation surveillance data collected in Belgium. (A) We here report RR values estimated for different age categories using a retrospective sliding window of four weeks. RR estimates are associated either with the “at least primary course completed” vaccination scheme (solid curves) or with the “boosted” vaccination scheme (dashed curves). Of note, in Belgium, hospitalised patients having had their booster jab were only registered as “boosted” at their admission from December 8, 2021. Therefore, we are only able to make the distinction between “primary course completed” and “boosted” vaccination schemes for the second half of the study period (15/12/22–28/02/22). Shaded polygons correspond to 95% confidence intervals associated with the “at least primary course completed” vaccination scheme (20). In addition, we also report (B) the progression of the Belgian booster vaccination campaign (shaded polygons *) and the evolution of the number of new hospitalisations in each slide of the retrospective sliding window of four weeks for the different age categories (curves), as well as (C) the temporal evolution of the relative detection frequency of the two main variants of concern (VOCs) circulating during that period in Belgium. (*) Because hidden on the graph by the progression of the booster vaccination campaign for younger age categories, the related progression for the 85+ is highlighted by a dashed curve from early 2022.