Impact of primary course COVID-19 vaccination on hospitalizations in Belgium, a Bayesian approach

Abstract Background The Belgian COVID-19 primary course vaccine rollout was prioritized for individuals aged 18-64 years with underlying health conditions (increased risk of severe COVID-19), over their peers of the same age. This led to a 32-day earlier vaccination of prioritized individuals. In this research, we assessed the impact of vaccine rollout prioritization on hospitalizations for COVID-19 in Belgium between April 1st 2021 and June 30th 2021. Methods The sample consisted of the Belgian population aged 18-64 years. Individuals were classified by their priority status according to underlying health conditions and by vaccination status, creating weekly changing cohorts. The hospitalization probability was estimated by cohort using a national COVID-19 hospital surveillance. Two counterfactual scenarios were considered: (1) a no vaccination scenario and (2) a scenario in which the vaccine administration occurred at random in both cohorts (a vaccine rollout without prioritization). We used a Bayesian model to jointly estimate cohort-specific hospitalization probability and scenario outcomes. Results Out of the sample of 5,448,534 individuals, 22.65 % was prioritized. By May 1st, vaccination coverage was 18% and 2% within the prioritized and the non-prioritized cohort. In scenario 2, coverage within the prioritized was 6% by May 1st. There were 6968 hospitalizations reported through the hospital survey. Our initial scenario-outcomes indicated a reduction in hospitalizations of 3203 (95%CI[2548;3858]) and 919 (95%CI[406;1433]) by the vaccination prioritization, compared to scenarios 1 and 2, respectively. Conclusions This research provides insights in the impact of vaccination in a pandemic context in Belgium. We report a significant reduction in simulated COVID-19 hospitalization by vaccine prioritization compared to at random vaccination and no vaccination at all. Currently, we are investigating the effect of age-dependent vaccination next to at random vaccination. Key messages • The prioritised vaccination campaign in Belgium led to a significant reduction in hospitalizations. • Prioritisation based on underlying health conditions is an important public health measure.