Background and Objectives: The safety and effectiveness of vaccines are among the key priorities in COVID-19 pandemic management. Moreover, evidence-based data regarding vaccine safety and immunogenicity can play an important role in building the trust of the community regarding vaccination. The aim of this study was to investigate the safety and immunogenicity of Pfizer-BioNTech vaccine among healthcare workers in one hospital, 21 days after first dose. Materials and Methods: This study was conducted in the Hospital of the Lithuanian University of Health Sciences between February and March 2021. Hospital employees who arrived to receive the second dose of the Pfizer-BioNTech vaccine 21 days after the first one were invited to participate in the study: they were asked to complete an anonymous adverse events questionnaire and were offered a SARS-CoV-2 IgG/IgM rapid test. The study was performed at a single point, 21 days after the first dose of the vaccine. Results: Data of 4181 vaccine recipients were analysed. The first vaccine dose was associated with a 53.6% incidence of adverse events, mainly local reactions. Adverse events occurred more frequently in younger participants and women. Moderate adverse events were experienced by 1.4% of the vaccine recipients; 6.2% were incapacitated. Of the 3439 participants who performed a rapid IgG test, 94.5% were positive for IgG antibodies after the first vaccine dose. Seroconversion rates were lower in participants older than 47 years. Conclusions: Despite 1.4% moderate adverse events, no safety concerns or anaphylaxis were identified. The Pfizer-BioNTech vaccine induced an immune response in the overwhelming majority of recipients after a single dose. Younger participants experienced adverse events and were positive for IgG antibodies more frequently than older counterparts. It is important to mention that this study specifically considered short-term safety and reactions following vaccination and that long-term adverse effects were not investigated in the study. Thus, future research into both long-term adverse reactions and immune system programming is essential.