Background: Coronavirus disease 2019 is a public health emergency and the development of an effective vaccine against SARS-CoV-2 became imperative. In context of development of a new drug or vaccine, it is necessary to maintain vigilance about the possible adverse effects that arise of their application. Objective: To report the ESAVI frequency of the COVID-19 Pfizer-BioNtech BNT162B2 vaccine in IMSS personnel of the OOADR west State of México Material and methods: Prior review and authorization by the research and research ethics committee, an analytical cross-sectional study was carried out in health and administrative workers of the IMSS OOADR west State of México who received the first and second dose of COVID-19 Pfizer-BioNtech BNT162b2 vaccine from January to April of 2021. The significance of differences between groups was assessed by Student’s t-test, chi square or Fisher’s exact test, and p-value 0.05 was considered statistically significant. Results: 12,264 workers received immunization, the total number of ESAVIs reported was 102 (0.8%). Comparing both groups (health and administrative workers), there were no differences between clinical characteristics of the population, except in the variable age (p 0.003) and history of DM2 (p 0.01), the main symptoms were headache, myalgia and arthralgia in both groups, without significant statistical difference. There were no errors in the administration of the vaccine and only six serious cases of ESAVI occurred and required hospital management. It should be noted that five (83%) of the six serious cases of ESAVI corresponded to health workers. Conclusions: The frequency of ESAVI was similar to that reported in other international series, the six cases of severe ESAVI were discharged due to improvement, concluding that the COVID-19 Pfizer-BioNtech BNT162b2 vaccine is safe and the frequency of side effects is very rare (0.08%). Key words: Adverse event following immunization, Vaccine, Immunization, Pfizer-BioNtech Covid-19.