Background: In collaboration with several Belgian universities, hospitals and institutes,
Sciensano, the Belgian Scientific Institute of Public Health, set up a consortium to
facilitate and streamline the organization of COVID-19 vaccination studies primarily in
immunocompromised populations. The immune response of COVID-19 vaccination was monitored
and studied in the context of the previously PICOV study (P2020/424), Nephro- VAC studies
(P2020/284 and P2020/312) and Lung-VAC study (P2021/182). The constant emergence of new
variants of concern (VOCs), which become increasingly better at escaping infection and
vaccine induced immune responses, together with waning immunity over time, warrant
additional vaccination rounds. This is especially true in immunocompromised populations.

In the current study, we want to continue monitoring SARS-CoV-2 specific immunity over
the next two years, encompassing several future vaccination campaigns.

Method: A non-commercial multicenter academic prospective cohort study will be conducted
in immunocompromised populations and healthy adults for two years. These healthy people
will be recruited from the previously organized PICOV-VAC study (members of nursing home
staff) (EudraCT 2021-000401-24) and REDU-VAC study (EudraCT 2021-002088-23) while the
immunocompromised participants will be recruited from the previously established
PICOV-VAC, REDU-VAC, Lung-VAC and Nephro-VAC cohorts from which historic clinic and
immunologic data is available. Participants will be sampled three times a year
independently of the vaccinations which will be administered through regular channels not
linked to the study itself.

Objectives: The main goal of this study is to measure levels of immunity in healthy
adults and immunocompromised participants three times a year. The primary objective is to
determine binding and neutralizing antibody levels against the epidemiologically
predominant SARS- CoV-2 variants of concern (VOC) and the wild type SARS-CoV-2 (Wuhan
strain). This will allow us to determine to which extent extra booster doses can or
cannot induce more (binding) and/or better (neutralizing) antibodies to different
variants as compared to peak responses achieved after previous vaccination doses and to
study waning of these responses after winter periods.

Secondary objectives include studying the vaccine induced immunity in more detail. This
includes the further characterization of the quality of the antibody response and the
measurement of the cellular immune response, amongst others.