Mass vaccination against the disease caused by the novel coronavirus (COVID-19) is a crucial step in slowing the spread of SARS-CoV-2 . The BioNTech/Pfizer (BNT162b2) vaccine has been shown to induce strong immune responses among the vaccinated population . Measuring SARS-CoV-2 anti-spike protein IgG levels is a clinically convenient way to estimate post-vaccination humoral immune responses, but only limited data exists about its short- and long-term dynamics . We present a longitudinal analysis of post-vaccination IgG levels in a cohort of 122 healthcare workers vaccinated with BNT162b2 with weekly follow-up until 35 days past the first dose and results of the first monthly follow-up after that for a subset of these . This prospective, multicenter cohort study consists of two periods for short-term and long-term evaluation of post-vaccination IgG levels . Tests were carried out on 666 samples from 122 participants, using in-house anti-spike 1 and anti-nucleocapsid IgG ELISA assays and a commercial, combined version of these . Participants with previous SARS-CoV-2 infection mount a quick immune response, reaching peak IgG levels two weeks after vaccination . In contrast, the corresponding IgG levels for previously uninfected participants increase gradually, changing abruptly after the booster dose . Overall higher IgG levels are maintained for the previously infected group 35-70 days after vaccination, and we observe age-dependence of immune response as well . Our results show a robust humoral immune response mounting gradually after the first vaccine dose for the uninfected group, and a much stronger immune response within 7-14 days after the first dose for the previously infected group.