Three Outbreaks of COVID-19 in a Single Nursing Home over Two Years of the SARS-CoV-2 Pandemic.

Older people in nursing homes (NH) have been hit particularly hard by the COVID-19 pandemic. We conducted a retrospective study of three outbreaks of COVID-19, occurring during the waves of the initial pre-Alpha, Delta and Omicron SARS-CoV-2 variants, in one NH in suburban Belgrade, Serbia. All staff and 95% residents were vaccinated in February 2021, mostly with BBIBP-CorV, and two thirds were boosted with a third dose in August 2021. COVID-19 was diagnosed by positive PCR and/or antigen test. After the first outbreak, 80 affected individuals were tested for SARS-CoV-2 specific antibodies. The first outbreak involved 64/126 (50.8%) residents and 45/64 (70.3%) staff, the second 22/75 (29.3%) residents and 3/40 (7.5%) staff, and the third involved 36/110 (32.7%) residents and 19/56 (33.9%) staff. Clinical presentation ranged from asymptomatic to severe, with severe cases referred to hospital ICUs. Deaths occurred only in residents, and the case fatality rate was 31.2%, 9.1% and 0%, respectively in outbreaks 1, 2 and 3. Specific IgG antibodies were detected in all 35 residents and 44 of the 45 staff, and higher IgG levels were detected in the residents (417.3±273.5) than in the staff (201.9±192.9, p<0.0001) despite a double difference in age (79.0±7.4 vs. 40.1±11.5 years). Outbreaks 2 and 3 involved four and 23 breakthrough infections, respectively. Older individuals mounted a good immunological response to SARS-CoV-2 infection and vaccination, which prevented significant mortality and severe morbidity in the subsequent outbreaks, despite a significant number of breakthrough infections.