Background: The effectiveness of administering booster or additional doses with vaccines adapted to new variants and subvariants is still a matter of debate.
Objective: To compare the cases of covid-19 in vaccinated 4th dose people with bivalent mRNA vaccines vs. cases of covid-19 in not vaccinated 4th dose people and assess the clinical-epidemiological differences and the relative effectiveness of this vaccine booster.
Methodology: An observational, longitudinal and prospective case series study of adult patients with covid-19 infections in general medicine from October 1, 2022 to October 1, 2023.
Results: 21 covid-19 infections in vaccinated people with 4th dose and 55 without 4th dose were included. The cases of covid-19 in vaccinated people with 4th dose differed in a statistically significant way compared to the cases of covid-19 in people without 4th dose in that they were older (67% vs. 13% > = 65 years); they had more chronic diseases (86% vs. 54%), being according to disease groups only significant in Genitourinary (8% vs. 21%); and presented fewer covid-19 symptoms of ENT (15% vs. 30%). The 4th dose vaccine effectiveness, calculated as: 1 – [Covid-19 cases incidence with 4th vaccine dose / Covid-19 cases incidence without 4th vaccine dose] × 100, was 60%.
Conclusion: In the context of general medicine in Toledo (Spain), there is a risk profile and a 4th dose vaccine effectiveness that together suggests that it is reasonable to maintain booster doses of the vaccine against covid-19 in the vulnerable population.
Keywords: COVID-19; SARS-CoV-2; Vaccine Effectiveness; Breakthrough Infection; Hybrid Immunity; Epidemiology; General Practice.