Characteristics and Vaccine Booster Effectiveness in Covid-19 Infections Using 15 Days Post-Booster Period as Baseline during the Omicron Wave in A General Medicine office in Toledo (Spain)

Background: Vaccine covid-19 booster effectiveness (VBE) timing is not clearly known Objective: To compare the cases of covid-19 in vaccinated booster people with a time of <15 days vs. >= 15 days from booster to infection diagnosis and assess their relative VBE. Methodology: An observational, longitudinal and prospective 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, 15 cases of Covid-19 breakthrough infections with booster shot <15 days (33%) with a mean time in days from booster to diagnostic covid-19 of 6 days, and 31 cases with booster > = 15 days (67%) with a time in days from booster to covid-19 diagnostic of 37 days of mean. Relative VBE <15 days (cases where it can be considered that the booster is not yet effective) vs. > = 15 days (cases where it can be considered that the booster is already effective) [1 - (Cases with vaccine Booster shot < 15 days) / (Cases with vaccine Booster shot > = 15 days) x 100] was 60%. Covid-19 cases with booster shot <15 days had been more vaccinated with 2 doses of ChAdOx1 nCoV-19 vaccine (Vaxzevria, Oxford / AstraZeneca) 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 <15 days provided 60% relative protection against symptomatic disease vs. > = 15 days. The results suggest that booster received <15 days provided early protection against SARS-CoV-2 infection of symptomatic Covid-19 of 60%. However, this result does not seem logical: this lower early risk may be transient and due to “vaccinated bias.”