[Brote de la COVID-19 en una residencia en ancianos correctamente vacunados. Influencia de la inmunidad híbrida en la carga viral, el riesgo de infección y el riesgo de progresión de la enfermedad].

OBJECTIVE:This study addressed a COVID-19 outbreak in a nursing home, where the residents were vaccinated with two doses of Comirnaty® and thirty of them had previously had the infection. Outbreaks in closed communities are opportunities to study the entire clinical spectrum of the disease and, as in this case, the effect of hybrid immunity on transmission, infection progression and viral load. METHODS:A descriptive and retrospective cohort study in the resident population was carried out. Attack rates were calculated for one of four stages of the disease: infection, symptomatic infection, hospitalization, and death. Relative risks (RR) were then estimated using simple and multivariate Poisson regression for each of these stages. RESULTS:The attack rate was 59% (56/95). The clinical spectrum was the same in both sexes. There was a notable protective effect of hybrid immunity against transmission (67%). In terms of progression, those with hybrid immunity had a lower risk of symptomatic infection. Nasopharyngeal viral load was significantly lower in individuals with hybrid immunity and asymptomatic individuals, supporting the idea of lower transmissibility in this group. Age was identified as a risk factor for disease progression. Molecular analysis identified the Delta B.1.617.2 variant in the patients and an air sample, supporting aerosol transmission in closed, poorly ventilated environments. CONCLUSIONS:This study provides a comprehensive view of an outbreak in a vaccinated nursing home, highlighting the importance of hybrid immunity. The results support the individual consideration of previous infection history when assessing the risk of COVID-19, contributing to the understanding of the evolution of the pandemic in the future.