Acute COVID-19 severity and impaired cognitive function up to 32 months after diagnosis: an observational study

Journal article

Background: Cognitive dysfunction (“brain fog”) is a commonly reported post-COVID-19 symptom. Leveraging data from five general population cohorts across four European countries (Estonia, Iceland, Norway, and Sweden), we assessed long-term prevalence of impaired subjective cognitive function among individuals diagnosed with COVID-19 by acute illness severity. Methods: The included cohorts consisted of adult participants recruited from March 2020 and followed with self-report measures of cognitive function and past COVID-19 infection (except one cohort consisting of clinically confirmed COVID-19 cases) through February 2023. In a cross-sectional analysis we contrasted the prevalence of impaired cognitive function among individuals with and without a COVID-19 diagnosis, overall and by illness severity up to 32 months post-diagnosis. We adjusted for age, gender, education, relationship status, binge drinking, body mass index, previous psychiatric diagnosis, number of chronic medical conditions, and response period. In a longitudinal analysis, we assessed potential changes in cognitive function scores before and after COVID-19 diagnosis. Results: The study population consisted of 153,841 participants (71% women), with 31,359 (20.4%) reporting a positive COVID-19 test. Overall, a COVID-19 diagnosis was not statistically significantly associated with increased prevalence ratio (PR) of impaired cognitive function (PR 1.30 [95% CI: 0.98–1.71]). Individuals bedridden due to COVID-19 for 1–6 days (PR 1.38 [95% CI 0.96–1.99]) or ≥ 7 days (2.59 [1.55–4.33]) had higher prevalence of impaired cognitive function compared to those never diagnosed, while individuals never bedridden had a lower prevalence to those never diagnosed with COVID-19 (0.89 [0.80–1.00]). These findings were corroborated in the longitudinal analysis where a pre- to post diagnosis decline in cognitive function was observed among individuals bedridden due to COVID-19 (p < 0.0001). Conclusions: The data indicates that a severe COVID-19 acute illness course is associated with impaired cognitive function up to 18–32 months after COVID-19 diagnosis.

Authors: Magnúsdóttir, I; Nygaard, AB; Hoffart, A; Murphy, G; Kõiv, K

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: BioMed Central
• Journal: BMC Medicine
• Volume: 24
• Issue: 1
• Article number: 311
• Publication date: 2026-04-11
• Acceptance date: 2026-04-02
• DOI: 10.1186/s12916-026-04856-2
• EISSN: 1741-7015
• ISSN: 1741-7015
• Language: English
• Keywords: Cohort; Population; Cognitive function; Post-COVID-19 condition; Brain fog; COVID-19; Long covid