Six years after SARS-CoV-2 emerged, the question of how long its effects persist, particularly after the milder Omicron variant, remains a pressing public health concern. A new study from the Faroe Islands, published June 16 in PLOS ONE, provides one of the most detailed clinical portraits yet of long COVID following Omicron infection, with follow-up extending to 13 months.
The study found that a substantial subset of people infected during the Omicron wave continue to experience a heavy symptom burden more than a year after their initial illness. Fatigue affected 97 percent of the long COVID group. Headache affected 77 percent. Problems with sleep, concentration, and memory affected 72 to 74 percent. And the odds of long COVID increased by 57 percent with each additional self-reported respiratory infection in the year following the initial bout.
The Faroe Islands, a self-governing territory of Denmark in the North Atlantic, offer a distinctive setting for epidemiological research: a small, genetically homogeneous population of roughly 54,000 people, managed through early intervention, widespread testing, effective contact tracing, and high public trust, resulting in low infection rates early in the pandemic and minimal societal disruption overall.
Researchers at the Department of Research, National Hospital of the Faroe Islands, and the University of the Faroe Islands recruited 200 individuals with confirmed Omicron infection who reported persistent symptoms, and 93 antibody-verified never-infected controls. After excluding 23 who had fully recovered by follow-up and 28 controls whose antibodies indicated prior asymptomatic infection, the final analysis included 177 long COVID cases and 65 never-infected individuals.
The long COVID group had a mean follow-up of 11 months (range 9.5 to 13 months). The groups differed at baseline: the long COVID group was younger and reported less daily medication use than the controls, making the symptom disparity more striking.
Fatigue and cognitive impairment dominate
Participants completed three standardized instruments: a 24-item symptom questionnaire, a mental impairment scale, and the Daily Fatigue Impact Score (D-FIS).
The total symptom score was strongly associated with long COVID. Each additional symptom raised the odds by 21 percent (adjusted odds ratio 1.21, 95 percent CI 1.13 to 1.30, p < 0.001). Sub-analyses revealed the strongest associations for fatigue, cognitive impairment, neurological symptoms, and symptoms from the cardiopulmonary and musculoskeletal systems.
The mental impairment questionnaire showed that 73 percent of the long COVID group reported problems with concentration, 72 percent with memory, and 71 percent with feeling stressed. Post-exertional fatigue affected 67 percent. These were not primarily emotional or depressive symptoms, the long COVID group scored distinctively higher on cognitive items than on mood items, while the control group showed a more even distribution across both domains. Each point increase on the mental impairment scale raised the odds of long COVID by 27 percent (aOR 1.27, 95 percent CI 1.14 to 1.42, p < 0.001).
The fatigue impact score, collected only from participants who reported fatigue (171 of 177 in the long COVID group, 23 of 65 in the control group), showed a similar pattern: each point increase raised odds by 27 percent (aOR 1.27, 95 percent CI 1.11 to 1.46, p < 0.001). Reduced alertness showed the strongest individual association.
A sex difference, and a blood signature
Women in the long COVID group reported consistently higher scores than men across all three questionnaires, and female sex was itself associated with higher odds of long COVID, an increase of 6 to 12 percent depending on the instrument. This difference was not seen in the never-infected group.
Blood analyses revealed a notable finding in women only: those in the long COVID group had lower white blood cell counts than never-infected women (mean total leukocytes 6.7 vs 7.2 x 109 per liter). A one-unit decrease in white blood cell count was associated with a 1.52-fold increase in the odds of long COVID in women (aOR 0.66, 95 percent CI 0.49 to 0.89, p = 0.006). The same pattern was seen for neutrophils, basophils, and lymphocytes.
The authors note, however, that very few women in the long COVID group had clinically defined leukopenia, neutropenia, or lymphopenia, and the clinical significance of the finding remains uncertain. No differences were seen in men, and no differences were found in acute phase reactants such as C-reactive protein, ferritin, or thrombocytes.
More infections after the infection
The long COVID group reported a mean of 1.8 self-resolved respiratory infections in the year following SARS-CoV-2 infection, compared with 0.7 in the never-infected group. Each additional reported infection raised the odds of long COVID by 57 percent (aOR 1.57, 95 percent CI 1.06 to 2.34, p = 0.025). This was not reflected in antibiotic prescriptions, there was only a non-significant trend toward higher antibiotic use, suggesting the infections were self-limiting viral illnesses.
The authors note two possible explanations. One is “immunity debt”: reduced exposure to common pathogens during lockdowns means the immune system has less practice dealing with them afterward. The other, more concerning, is “immunity theft”: the hypothesis that COVID-19 itself may impair immune function and increase susceptibility to subsequent infections. “Evidence supporting this remains preliminary,” the study acknowledges, “though it may align with trends observed in our findings of increased infectious episodes, especially among women.”
What this tells us about Omicron-era long COVID
The study adds to a growing body of evidence, including large case-control studies from Japan and China, that Omicron infection carries a meaningful risk of persistent symptoms, even though the acute phase of the illness is generally milder than with earlier variants. The Faroe Islands data are notable for the long follow-up window (up to 13 months), the use of antibody-verified never-infected controls, and the detailed symptom characterization across three separate instruments.
The study’s main limitation is its moderate sample size (177 cases, 65 controls) and the fact that participation rates differed between groups (68 percent for cases, 83 percent for controls). The findings are also specific to a small, genetically homogeneous island population with a highly vaccinated background and well-managed pandemic response, which may limit generalizability.
The authors concluded: “One year after Omicron infection, a subset of people continue to experience a substantial symptom burden, particularly fatigue, cognitive impairment, and reduced mental well-being, and a higher frequency of intercurrent infections.”
Source:
- G. Helmsdal et al., “Persistent symptoms, cognitive impairment, and clinical predictors of long COVID one year after Omicron infection: A clinical case-control study from the Faroe Islands,” PLOS ONE (2026). DOI: 10.1371/journal.pone.0351564