Study Evaluating SARS-CoV-2 (COVID-19) Humoral Response After BNT162b2 Vaccine in Immunocompromised Adults Compared to Healthy Adults

The primary endpoint of this study is to compare the humoral response (titre and neutralizing capacity of induced antibodies) against SARS-CoV-2 following vaccination with BNT162b2 (Pfizer BioNTech) in immunocompromised persons, in comparison to healthy subject. Secondary objectives are to evaluate the humoral response in the nasal mucosa, and the capacity of antibodies to neutralize emerging variants of concerns and to prevent COVID-19.

The serious, even fatal, forms of COVID-19 preferentially affect elderly and fragile subjects. Among these populations at risk, people who are immunocompromised (either by a disease and / or its treatment) have a theoretical risk of responding less well to a preventive vaccination. The main objective of this study aims to compare the vaccine response of immunocompromised people with healthy subjects (non-immunocompromised), i.e. to assess the serum humoral response (titre and neutralizing capacity of the antibodies induced) following vaccination with ComirnatyTM (i.e. BNT162b2, an anti-SARS-CoV-2 vaccine from Pfizer BioNTech) in immunocompromised persons in comparison to healthy subjects (non-immunocompromised). Secondary objectives are as follows: * To evaluate the antibody response in the nasal mucosa (titre and neutralizing capacity of the antibodies induced, collected by means of a nasopharyngeal swab) following vaccination with ComirnatyTM in immunocompromised people as compared to healthy subjects (vaccinated either with two doses of ComirnatyTM or, in a subgroup, with one dose of Astra Zeneca's VaxzeriaTM followed by one dose of ComirnatyTM). * Evaluate the serum and mucosal antibody response (titre and neutralizing capacity of the antibodies induced) against emerging strains of SARS-CoV-2 (so-called Alpha, Beta, Gamma, Delta strains). * Evaluate the post-vaccination clinical protection against the risk of COVID-19 infection (incident cases after vaccination).