The COVID-19 pandemic disproportionately affected older adults in nursing homes (NHs). This research investigated three distinct COVID-19 outbreaks within a single NH in suburban Belgrade, Serbia, spanning two years and coinciding with the dominance of pre-Alpha, Delta, and Omicron SARS-CoV-2 variants. The study aimed to characterize infection, transmission, and outcomes among residents and staff during each outbreak, paying special attention to sex-related differences. Given that the first outbreak preceded widespread vaccination and the subsequent two followed a vaccination campaign, the study also sought to compare outbreak characteristics before and after vaccination and assess vaccine effectiveness. The high vulnerability of this population to severe COVID-19 and the potential for significant mortality underscored the urgent need to understand the dynamics of the virus in these settings, particularly in light of evolving viral variants and the implementation of vaccination strategies. Previous studies had demonstrated the effectiveness of vaccination in reducing transmission and severity of COVID-19 in older adults, but outbreaks and severe outcomes continued to occur in NHs, even with high vaccination rates, highlighting the challenges posed by constantly emerging viral mutations and potential waning immunity. This study, therefore, provides critical insights into the effectiveness of vaccination strategies, the impact of emerging viral variants, and the overall clinical presentation and outcomes of COVID-19 in a real-world setting of a long-term care facility.

Existing literature highlighted the devastating impact of COVID-19 on nursing home residents, with high infection and mortality rates reported globally. Studies in the UK, Spain, and Ireland demonstrated substantial COVID-19 outbreaks in nursing homes, with case fatality rates ranging from 25% to 35%. These studies also pointed to factors such as facility size, community incidence, and the presence of comorbidities as significant risk factors for infection and mortality. While vaccination campaigns significantly reduced outbreaks and mortality rates in some settings, studies also noted that outbreaks still occurred even with high vaccination coverage, emphasizing the continued vulnerability of nursing homes to the virus, particularly with the emergence of new variants. A significant Israeli study, utilizing national surveillance data, illustrated the high effectiveness of the Pfizer-BioNTech mRNA vaccine in older adults, but also noted ongoing outbreaks in nursing homes despite widespread vaccination. Similarly, reports from multiple nursing homes across different periods showed declining effectiveness of mRNA vaccines against SARS-CoV-2 infection with the emergence of the Delta variant, underlining the need for further research into the dynamics of COVID-19 outbreaks in this vulnerable population.

This retrospective observational study analyzed data from three COVID-19 outbreaks in a private nursing home near Belgrade, Serbia. The nursing home had a capacity of 160 residents, with occupancy ranging from 60% to 72% during the outbreaks. The study population included all residents and staff affected during the three outbreaks. Data collected from medical records included demographic information (age, sex), comorbidities, immunization status, clinical symptoms, treatment, hospitalization status, and clinical outcomes. SARS-CoV-2 infection was confirmed using PCR and/or antigen tests. A subset of individuals from the first outbreak underwent serological testing to assess SARS-CoV-2 specific IgM and IgG antibody levels. The study adhered to the Declaration of Helsinki principles, and informed consent was obtained for serological testing. The study timeline coincided with the waves of pre-Alpha (November 2020), Delta (November 2021), and Omicron (January 2022) variants. Epidemiological measures implemented in the nursing home aligned with Serbian Ministry of Labor guidelines, encompassing personal protective equipment (PPE) for staff, isolation of infected residents, and strategies to limit transmission. Statistical analysis included Shapiro-Wilk and Kolmogorov-Smirnov tests for normality, followed by appropriate parametric (t-test, ANOVA) or non-parametric (Mann-Whitney, Kruskal-Wallis) tests. The level of significance was set at 5%. Serological testing for SARS-CoV-2 specific antibodies (IgM and IgG) was performed using the MINI VIDAS analyzer and VIDAS SARS-CoV-2 IgM and IgG II kits (BioMérieux). Results were expressed as indices, with values ≥1 considered positive. IgG indices were converted to Binding Antibody Units/ml (BAU/ml).

A total of 189 COVID-19 infections were recorded across the three outbreaks: 122 (64.6%) in residents and 67 (35.4%) in staff. Outbreak 1 (pre-vaccination, pre-Alpha variant) had the highest number of cases (109), disproportionately affecting staff (70.3%) compared to residents (50.8%). Outbreak 2 (Delta variant) involved fewer cases (29.3% residents, 7.5% staff), while Outbreak 3 (Omicron variant) showed roughly equal distribution among residents (32.7%) and staff (33.9%). Resident age was significantly higher than staff age across all outbreaks. Most infected individuals were female, though this difference was not statistically significant. However, a higher proportion of males were affected in unvaccinated staff during Outbreak 1. Clinical presentations ranged from asymptomatic to severe. Deaths occurred only among residents, with case fatality rates (CFR) of 31.2%, 9.1%, and 0% for Outbreaks 1, 2, and 3, respectively. The most common comorbidities were arterial hypertension, type II diabetes mellitus, and dementia. Hematological analysis revealed that elevated leukocyte, neutrophil counts, and NLR, along with increased CRP and LDH levels were associated with poorer clinical outcomes. In outbreak 3, neutrophil and thrombocyte counts were significantly higher in males, while lymphocyte counts were lower. In a subgroup analysis from outbreak 1, all residents (35/35) and most staff (44/45) had detectable IgG antibodies, with significantly higher levels in residents. Outbreaks 2 and 3 predominantly involved breakthrough infections (vaccinated individuals), primarily after BBIBP-CorV vaccination. Several reinfections occurred, most often after BBIBP-CorV vaccination, but these were clinically mild without severe cases or deaths. Analysis of mortality rates showed a significant increase in the last trimester of 2020 compared to previous years, attributable to COVID-19. The timeline analysis showed that the time taken for each variant to enter the NH decreased as the variants became more infectious and vaccine resistance grew.

This study's longitudinal observation of three COVID-19 outbreaks within a single nursing home offers unique insights into the impact of different SARS-CoV-2 variants and the effect of vaccination. The high CFR (31.2%) in the pre-vaccination outbreak aligns with previous research. Subsequent outbreaks, following the vaccination campaign, showed a significant reduction in mortality, demonstrating the vaccine's effectiveness in preventing severe disease and death, particularly in the Omicron wave. The substantial number of breakthrough infections, especially with the Omicron variant, highlight the challenges posed by highly transmissible variants and potential waning immunity from vaccination. The study's findings on immune responses suggest a well-preserved immune response to the virus in older adults after both natural infection and vaccination, particularly concerning IgG antibodies. The association between elevated inflammatory markers (leukocytes, neutrophils, CRP, LDH) and severe disease aligns with previous studies and suggests these could serve as potential prognostic indicators. The study's findings on the relatively low effect of sex in a population of older adults are consistent with prior research showing the diminishing role of sex as a risk factor with increasing age.

This study demonstrates that while vaccination significantly reduced the severity and mortality of COVID-19 in a nursing home setting, it did not entirely prevent outbreaks, particularly with highly transmissible variants like Omicron. The strong humoral immune response observed in older adults after infection and vaccination emphasizes the importance of vaccination in preventing severe disease. Future research could focus on longer-term serological follow-up, investigation of the long-term effects of infection and vaccination in this population, and the evaluation of different vaccine types and strategies to optimize protection against emerging SARS-CoV-2 variants in nursing homes.

The study's limitations include the lack of serological data beyond the first outbreak, preventing a complete assessment of long-term antibody responses. Additionally, hematological data were limited to residents only, hindering direct comparisons with staff. Furthermore, the study might not be fully generalizable to other nursing homes or populations with different vaccination strategies or prevalent comorbidities.