Background: The effectiveness of COVID-19 vaccines in preventing serious infection and death is established, but their protection against infection is less certain. Additionally, their effectiveness diminishes over time. Furthermore, the evolution of the effectiveness of different booster doses of the vaccine against COVID-19, to prevent symptomatic infection in real life during the pandemic and the subsequent endemic, is not clearly documented.
Objective: To compare the effectiveness of the 3rd and 4th vaccine boosters against COVID-19 in preventing symptomatic COVID-19 infection during both the pandemic and the subsequent endemic phase.
Methodology: A comparative secondary analysis of the vaccine’s effectiveness against symptomatic COVID-19 infection (calculated as: 1 – (COVID-19 cases with vaccine doses / COVID-19 cases without vaccine dose) × 100) based on a prospective study from December 2021 to October 2023 in a general medicine office was conducted. The first booster dose was administered with monovalent mRNA vaccines, and the second booster with bivalent mRNA vaccines.
Results: From December 2021 to February 2022, the effectiveness of the primer vaccine booster was 60% when administered >= 15 days versus <15 days before infection, and 36% when administered >= 29 days versus < 29 days before infection. From October 2022 to February 2023, the effectiveness of the vaccine’s 4th dose was 84%. From October 2022 to October 2023, the effectiveness of the 4th dose of bivalent mRNA vaccine in preventing reinfections was 30%.
Conclusion: In the general practice setting in Toledo, Spain, the effectiveness of the first booster with mRNA vaccines against SARS-CoV-2 primary infection and symptomatic COVID-19 waned over time, but protection remained high with the second bivalent booster. However, the booster vaccine’s effectiveness is more modest in preventing symptomatic reinfections. Overall, completing the booster vaccination is worthwhile.
