Study cohort of SARS-CoV-2 test-positive and test-negative individuals
The study cohort is based on the EFTER-COVID survey, which was established in order to study the degree of, nature of and duration of possible post-acute symptoms and health problems among SARS-CoV-2 test-positive individuals using test-negative as a reference group27. Test-positive and test-negative individuals were invited to participate using information on SARS-CoV-2 PCR test results from the national COVID-19 surveillance system at the Statens Serum Institut28,29. Since 1968, the Danish Civil Registration System has assigned a unique personal ID-number (the CPR-number) to every person resident in Denmark30, which is used as a personal identifier in all Danish national registers including the COVID-19 surveillance system. Invitations to participate in EFTER-COVID were sent to test-positive and test-negative individuals using E-Boks, a national digital secure two-way communication system between citizens and authorities in Denmark, via the CPR-number31. E-Boks usage is mandatory for all inhabitants in Denmark aged 15 years or older, unless exempted (in 2022 used by 92% of the relevant age group)27. In general, all individuals who received a positive PCR-test result for the first time were invited. Test-negative individuals with no prior positive test result were density matched on test-date with a test-negative/test-positive individual ratio of 3:2 in anticipation of a lower response rate among test-negatives27.
The EFTER-COVID survey
Questionnaires were sent out from August 1, 2021 and cover SARS-CoV-2 test dates from September 1, 2020 to February 21, 2023. Individuals with test dates after April 2, 2021, were randomly assigned to one of four prospective tracks focusing on physical health, mental health, cognitive deficits and fatigue, respectively27. Every participant was asked to fill in a baseline questionnaire, and if they consented, they received follow-up questionnaires approximately 2, 4, 6, 9, 12 and 18 months after the test. Participants were enrolled 1 month after their test date except for a smaller fraction of participants, who received special 2- or 4-months baseline questionnaires during project start-up.
The baseline questionnaire contained questions regarding acute symptoms (symptoms in the period one week before and until four weeks after the test), and issues such as weight, height, smoking and drinking habits, education level, employment, physical fitness, selected comorbidities, selected disease diagnoses and general health conditions27. Only participants who had filled in both the baseline questionnaire and at least one follow-up questionnaire were included in the present study.
Outcome
In the present study, we used data from the EFTER-COVID track focusing on cognitive problems, which were evaluated using the “Cognitive complaints in bipolar disorder rating assessment” (COBRA) questionnaire32. In the baseline questionnaire, participants were asked about pre-existing (within the six months prior to test) cognitive function. In the follow-up questionnaires, participants rated cognitive function in relation to the past 14 days from when they responded.
COBRA is a self-reported instrument which collects cognitive complaints related to processing speed, working memory, verbal learning and memory, attention/concentration, executive function, and mental tracking32. It consists of 16 questions about cognitive difficulties in daily life scenarios e.g., “Do you find it hard to concentrate when reading a book or a news-paper?”, “Do you have the feeling that you do not finish what you begin?”33. The COBRA questionnaire (i.e., the 16 questions) exists in several languages and are freely available through the International Society for Bipolar Disorders’ website at www.isbd.org/cognitive-assessment33. All items/questions are rated using a four-point scale, 0=never, 1=sometimes, 2=often, and 3=always. Scores are added together (0–48), and the higher the score, the more subjective complaints32. In our main analysis, we analyzed scores as count data. In a supplementary analysis, we dichotomized scores into 34.
Covariates and other data sources
From the Danish National Patient Register (DNPR), we obtained information on hospital contacts (outpatient, inpatient and emergency departments) for the last five years before the test date to calculate Charlson Comorbidity Index (CCI), (Supplementary table 1), pre-existing psychiatric disorders as far back as 2005 (Supplementary table 2) and severity of the SARS-CoV-2 infection, classified as non-hospitalized and hospitalized (hospitalized within 14 days after until at most 2 days prior to the positive test). Individuals who tested positive more than two days after hospital admission were censored at the date of SARS-CoV-2 test as they were considered to have acquired the infection in the hospital (n = 32). Healthcare workers were identified using the authorization register35, vaccination status was obtained from the Danish COVID-19 vaccination register (https://miba.ssi.dk/forskningsbetjening/ansoeg-om-data-via-forskerservice/tilgaengelig-data) and virus variant periods defined according to periods of predominance in Denmark (Alpha; March 15 to June 30, 2021, Delta; July 15 to November 15, 2021, Omicron; January 1, 2022 and onwards)36. Individuals whose positive or negative test date fell within the transitional periods were not included in the variant stratified analysis. From the baseline questionnaire we included self-reported information on height, weight, education level and employment (Supplementary Table 3).
Statistical analysis
We used Poisson mixed effects models to estimate Score Ratios (SRs) comparing mean COBRA scores among test-positive versus those among test-negative individuals for the pre-test period (0-6 months before test) and at different time-points of follow-up, for those who had filled in the follow-up questionnaire at that specific time-point (i.e., 2-, 4-, 6-, 9-, 12-, and 18 months after test). These models took the following fixed effects into account: age group, sex, obesity (yes/no), CCI, healthcare occupation, employment status, dominant virus variant at test date, vaccination status, and education level. The individual identifier (the CPR number) was included as a random effect to account for the repeated and presumably not independent measurements of COBRA scores per individual over time (months after SARS-CoV-2 test). 95% confidence intervals (CI) were calculated using the Wald method.
We furthermore calculated SRs for the combined 2–18 months after test by including and aggregating all COBRA scores per individual i.e., not taken time since test into consideration, while controlling for independence of the observations by including the random effect, the CPR number.
In addition, we explored factors modifying the association between infection and COBRA scores by stratifying the SRs for the combined 2–18 months of follow-up on age group, sex, obesity (yes/no), CCI, healthcare occupation (no; yes, in frontline; yes, but not in frontline), employment status, dominant virus variant at index test date, vaccination status, education level and severity of infection.
In a supplementary analysis, we used logistic mixed effects models to estimate odds ratios (ORs) with 95% CIs by comparing odds for cognitive complaints (COBRA score >=15) vs no cognitive complaints (score
All statistical analyses were carried out using R version 4.3.0. Forest plots were generated using the R-package called forestploter37.
A posteriori analysis
We repeated the main analysis only including the subgroup of individuals who had completed all the follow-up questionnaires. This was done to evaluate the impact of survey attrition whereby individuals in good cognitive health respond to fewer follow-up questionnaires than individuals who experience cognitive issues, and are thus more motivated to participate.
Ethical approval
This study was performed as a surveillance study as part of the governmental institution Statens Serum Institut’s (SSI) advisory tasks for the Danish Ministry of Health. SSI’s purpose is to monitor and fight the spread of disease in accordance with section 222 of the Danish Health Act. According to Danish law national surveillance activities conducted by SSI does not require approval from an ethics committee. It was approved by the Danish Governmental law firm and SSI’s compliance department that the study is fully compliant with all legal, ethical, and IT-security requirements and there are no further approval procedures regarding such studies.
Participation in the study was voluntary. The invitation letter to participants contained information about their rights under the Danish General Data Protection Regulation (rights to access data, rectification, deletion, restriction of processing and objection) and the type of information about them which might be processed (including registry data). Accessing and filling out the questionnaire after receiving the above information was considered informed consent to participate from the participant’s side. Participants could leave the survey at any time.
Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.
