At the beginning of the fifth SARS-CoV-2 wave in Israel, with the B.1.1.529 (Omicron) variant, the effectiveness of the third SARS-CoV-2 BNT162b2 vaccine (Pfizer-BioNTech) against Omicron was questioned. During the fifth wave, in January 2022, the Israeli Ministry of Health authorized a fourth BNT162b2 dose for individuals aged 60 years or older (a third dose was authorized for such individuals in July 2021, during the fourth SARS-CoV-2 wave).

Both humoral and cellular anamnestic responses are significant for protective immunity against SARS-CoV-2. In the current study, the responses in elderly people before and after a fourth vaccine dose of BNT162b2 were compared to those of individuals immunized with three vaccine doses. Although a boost effect was observed, the high response following the third administration questions the necessity of an early fourth boost.