Immunogenicity of BNT162b2 vaccine after two and three doses in health personnel and institutionalized elderly people not infected with SARS-CoV-2

Objective: The aim of our research was to compare the evolution of the immune response induced by the BNT162b2 vaccine after the administration of two and three doses in healthcare personnel and in institutionalized elderly people (>65 years of age) without previous SARS-CoV-2 infection.Material and methods: A prospective observational study was carried out on a convenience sample made up of health workers and institutionalized elderly people, measuring antibodies against S and N proteins of SARS-CoV-2 two and six months after receiving the second vaccine dose, as well as two months after receiving the third dose.Results: A significant reduction of the anti-S humoral immune response was reported six months after the second dose of vaccine in both health workers and residents. The administration of a third dose of vaccine induced a significant increase in this antibody response in both investigated groups reaching a similar proportion of responders two months after this third dose.Conclusions: Humoral immunity induced by two doses of the BNT162b2 vaccine in persons without prior SARS-CoV-2 infection wanes over time. The administration of a third dose significantly increases anti-S antibodies being highly recommended, especially in people over 65 years of age.