Background: When the highest vaccine COVID-19 booster effectiveness (VBE) is obtained is not clearly known.
Objective: To compare the cases of COVID-19 in booster vaccinated people with a time of <29 days vs. ≥ 29 days from booster to infection diagnosis and assess their relative VBE.
Methodology: An observational, longitudinal and prospective case series study of adult patients with COVID-19 breakthrough infections in booster vaccinated people, in general medicine and for the period December 2021 to February 2022, during the omicron variant contagion wave.
Results: Forty-six cases were included, 28 cases of COVID-19 breakthrough infections with booster shot <29 days (61%), with a mean time from booster to COVID-19 of 12 days (1-25 days), and 18 cases with booster ≥ 29 days (39%), with a mean time from booster to COVID-19 of 50 days (29-84 days). Relative VBE ≥ 29 days before infection [1 - (Cases with vaccine booster shot ≥ 29 days) / (Cases with vaccine booster shot <29 days) x 100] was 36%. No VBE found with shot <29 days vs. booster shot ≥ 29 days. COVID-19 cases with booster shot ≥ 29 days had shorter duration of symptoms, were more socio-health workers, and had more sick leave. COVID-19 cases with booster shot ≥ 29 days had been more vaccinated with 2 doses of Pfizer-BioNTech plus booster of mRNA-1273 vaccine (Spikevax, formerly covid-19 Vaccine Moderna).
Conclusion: In the general practice setting in Toledo, Spain, from December 1, 2021 to February 28, 2022, at the peak of omicron infections, booster after a period of <29 days had relative VBE of 36% against symptomatic disease vs. <29 days; but, our cohort was too small and the follow-up time was too short to allow precise determination of vaccine effectiveness.
COVID-19, SARS-CoV-2, Vaccine effectiveness, Breakthrough infection, General practice