Background
; We investigated the real-world effectiveness of a heterologous versus homologous primeboost
; vaccination scheme against severe clinical outcomes of SARS-CoV-2 Omicron
; infection among hospitalized patients in Belgium, and if this effect is homogenous across
; different Omicron sub-lineages.
; Methods
; Observational data from existing national surveillance systems and administrative sources
; were leveraged to estimate a causal effect. A Directed Acyclic Graph was formalized to
; identify confounding factors. The target population consisted of adult COVID-19 patients
; admitted to a Belgian hospital after the start of the booster vaccination campaign. Only
; patients with a SARS-CoV-2 Omicron infection (BA.1, BA.2, or BA.4 and BA.5 combined
; (BA.⅘) sub-lineages) confirmed by whole genome sequencing and vaccinated with a
; homologous (mRNA for prime/mRNA for boost) or heterologous (viral vector for
; prime/mRNA for boost) prime-boost vaccination schedule were included in the study
; population. An adjusted logistic model was build and g-computation was executed to
; estimate counterfactual risks of different clinical severity outcomes (intensive care unit (ICU)
; transfer, in-hospital mortality, and severe COVID-19 – defined as experiencing an acute
; respiratory distress syndrome (ARDS) event, or an ICU transfer, or in-hospital mortality) for
; patients that received a homologous versus heterologous prime-boost vaccination, and to
; contrast those. The role of the Omicron sub-lineage as an effect modifier was investigated by
; estimating and contrasting these counterfactual risks across different sub-lineages.
; Results
; 756 patients (499 homologous, 257 heterologous prime-boost vaccination) were included in
; the study population. No significant difference in counterfactual risks for severe COVID-19
; (RD=-0.04, 95%CI [-0.10–0.02]), ICU transfer (RD=-0.03, 95%CI [-0.07–0.02]), or inhospital
; mortality (RD=0.00, 95%CI [-0.05–0.04]) was found between homologous versus
; heterologous prime-boost vaccination, and no significant effect modification of the Omicron
; sub-lineage could be observed.
; Conclusions
; Our results suggest that there is no difference in protection against severe disease among
; hospitalized COVID-19 patients offered by the contrasted prime-boost vaccination schemes.
; These results can inform vaccine policy, and contribute to knowledge used for the
; development of strategies for booster campaigns during the continuous COVID-19 crisis.