Fourth BNT162b2 COVID-19 Vaccine Dose

The Omicron variant of concern (VOC) is currently rapidly spreading worldwide, with extremely high transmission rates, with an estimated R of \>3. the investigators now have preliminary, yet unpublished data, showing slow waning of the immune response after the third dose of the BNT162b2 mRNA vaccine within 4 months after this dose . While these data would not have been worrisome in the Delta VOC era, this may be different with the emergence of the Omicron VOC. These data raise the question of when and will a 4th dose be needed to cope with the emergence of Omicron. However, if a have reached the maximal effect of the current vaccine has been reached against Omicron, with a third dose, will a 4th dose have any added value? Here, the investigators will study the potential immunogenicity of a 4th dose, together with assessing safety and effectiveness in preventing infections