Background The COVID-19 pandemic continues to be a major worldwide health problem. The present study aims to contribute to surveillance of the immune and clinical response of vaccines to SARS-CoV-2. Methods Observational study of medication on acquired immunity and effectiveness of vaccines. Population: 620 workers in the health service of Almansa (Spain). Representative sample of 150 individuals. Sociodemographic, clinical, and epidemiological data and samples were recorded to determine anti- SARS-CoV2 serum IgG levels 6 and 9 months after vaccination with Pfizer. Results Mean age 46.45 years; 76% women; 85.1% working in a hospital. 19.3% had had COVID-19 in the year prior to vaccination. 96.7% were fully vaccinated with Pfizer/BioNTech. At 6 months, 100% seropositivity and mean IgG levels of 3017.2 AU/ml. Significant variations in IgG levels in individuals with prior COVID-19 infection and smokers. At 9 months, 99.3% remained seropositive; 2.8% infected after vaccination. The repeated measures analysis showed a difference in means of 669.0 AU/ml (significant decrease in IgG levels of 28.9%). Conclusion Antibody levels remained positive 6 and 9 months after vaccination, although IgG levels were found to decay.