A SARS-CoV-2 Outbreak Among Nursing Home Residents Vaccinated with a Booster Dose of mRNA COVID-19 Vaccine

: This study describes a SARS-CoV-2 outbreak caused by the Delta (B.1.617.2) variant in a nursing home in Central Italy during October-November 2021. Trained interviewers collected data from residents, staff, and administration officers with an agreed informed consent procedure. Thirty-two (44.5%) out of 72 residents (median age 89 years) and six (26.1%) of 23 healthcare workers were found to be infected with SARS-CoV-2. Infections occurred more often among residents with a higher index of independence in daily living activities, suggesting an increased risk for those with more interactions. Twenty-five infected residents (78.1%) received the booster dose of mRNA anti-COVID-19 vaccine > 7 days before SARS-CoV-2 onset. Half of the infected residents had mild symptoms, and only three required hospitalisation, one of whom died from COVID-19 complications. The study underlines the effectiveness of a booster dose in providing a high protection against severe disease and hospitalisation even among vulnerable individuals infected with the Delta variant of concern.