The aim of the study is to assess the immunogenicity of a 4th dose, and its durability. This will be measured by following IgG, IgA, Pseudoneutralization assays, microneutralization, avidity, T-cell activity and B-cell repertoire and comparing them to a matched control group, who are participating in the Sheba COVID Cohort study. The investigators will also assess safety and vaccine effectiveness by active surveillance of adverse events and by following incidence of SARS-CoV-2 infections.~This is a prospective intervention study, to test the effect of a 4th dose, by comparing the immune response before and after the 4th dose, given to 150-200 volunteers, as well as comparing their responses with a control group of individuals vaccinated with 3 doses but without the 4th. All study participants would be health care workers from Sheba medical Center, who are participating in the Sheba COVID Cohort study and have a serology test from the previous 3 months. Participation in the study will be confidential and will not be disclosed to the worker's direct supervisor. For this study, the investigators will recruit 150-200 volunteers, who received the 3rd dose at least 4 months previously, and have a known serology history (showing an immune response (even if just a low response) to the three previous doses, but with a recent relatively low IgG (below 700 BAU). Volunteers will tested before and after vaccination with a 4th dose, and followed for 6 months.~As controls, a sub-cohort of similar HCW, who are recruited to the Sheba COVID Cohort study (IRB 8008-20) and are followed monthly with serology tests, and are not receiving the 4th dose. The control group, all of whom signed an informed consent and allowed blood samples to be used for further immunologic studies, will be matched by age, gender, time from 3rd vaccine dose and IgG titers, and will be followed similarly, as by the original 8008-20 protocol.~On recruitment, volunteer will:~Receive a detailed explanation and sign the informed consent form (appendix ICF)~Fill an initial inclusion/ exclusion criteria questionnaire.~Fill a general comorbidity questionnaire (Appendix Q1), additionally they will be screened for COVID-19 symptoms such as fever, cough, anosmia~Have up to 40cc blood drawn for all serology and cellular immunity tests.~Perform a PCR for SARS-CoV-2 test~Receive the 4th dose of BNT162b2 30µg.~Will have a physician checkup and followup for 15 minutes after receiving the dose.~Six additional visits will follow as described in the research timeline:
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