A third dose of the Pfizer–BioNTech mRNA COVID-19 vaccine BNT162b2 (Comirnaty) in health-care workers in Greece administered 9 months after a second dose caused a remarkable increase in the concentration of SARS-CoV-2 anti-S1 RBD antibodies, which declined substantially 9 months after two doses of the BNT162b2 vaccine. The gene discussed is PSMD1; the disease is COVID-19.