Persistent symptoms, cognitive impairment, and clinical predictors of long COVID one year after Omicron infection: A clinical case–control study from the Faroe Islands

BACKGROUND: Six years since the emergence of SARS-CoV-2, the newer variants of the virus continue to have long-term health effects. OBJECTIVES: The aim of the study was to investigate persistent symptoms, cognitive impairment, and clinical and paraclinical predictors of long COVID in individuals infected during the Omicron wave. METHODS: We conducted a clinical case-control study including participants with persistent symptoms up to 13 months after confirmed SARS-CoV-2 Omicron infection (long COVID or LC group) and antibody-verified never-infected controls (NI group). RESULTS: A total symptom score based on a 24-item questionnaire was strongly associated with increased odds of long COVID (adjusted odds ratio (aOR) 1.21, 95% CI 1.13-1.30, p < 0.001). Sub-analysis showed particularly strong associations for fatigue, cognitive impairment, neurological symptoms, and symptoms from the cardiopulmonary and musculoskeletal systems. Both mental impairment and fatigue independently predicted long COVID (aOR 1.27, 95% CI 1.14-1.42, p < 0.001, and aOR 1.27, 95% CI 1.11-1.46, p < 0.001, respectively). Additionally, a higher number of self-reported infections during the follow-up period increased the odds of long COVID (aOR 1.57, 95% CI 1.06-2.34, p = 0.025), though this was not reflected in antibiotic use. Finally, blood analyzes showed that lower white blood cell counts were associated with increased odds of long COVID in women, but not in men, however the clinical significance of this finding remains uncertain. CONCLUSIONS: One year after Omicron infection, a subset of people continue to experience a substantial symptom burden, particularly fatigue, cognitive impairment, and mental well-being, and a higher frequency of intercurrent infections.