IDcare was performed in Skåne (Scania), the southernmost of Sweden’s 21 counties. In 2022, the population of Skåne was approximately 1.4 million inhabitants, corresponding to 13% of the country’s total population [45]. Except for the Stockholm region, which includes the Swedish capital, Skåne was the most densely populated region in the country, with 120 inhabitants/km2 [45]. The region consists of both urban and rural areas and is dominated by agricultural land. There are 21 large localities with more than 10 000 inhabitants, but only two had more than 100 000 inhabitants, with Malmö as the biggest city with a population of about 300 000. About 24% of the inhabitants in 2022 were born outside Sweden, compared to 20% in the country as a whole [45]. The county council of Skåne is called Region Skåne.

Sweden has a welfare system and health care is mainly funded by taxes, with only small out-of-pocket costs. The system is decentralized where health care, treatment, rehabilitation, and specialized medical care in inpatient and outpatient settings or in primary care centres, as well as physician home care, are provided by 21 county councils and regulated by the Health Service Act [46]. This act is fundamental for equality reasons as it establishes the right to equal access to health care for all Swedish residents. Sweden has a long tradition in population-based registers with information on an individual level. These registers are compiled by government agencies or other organisations. Several cover the health care and services provided in Sweden. Those that are related to health care and services acts [46,47,48] are kept by The National Board of Health and Welfare and are used for analyses and development of health care and services [49]. Besides these, there is a range of Swedish National Quality Registers (NQR). These are usually initiated and maintained by professions in health care and cover a certain group of patients with a specific diagnosis or treated using a certain procedure, for example hip fractures or preventative care for elderly. These quality registers also contain data on an individual level and include patient problems, medical interventions, and outcomes after treatment within all health care production. However, whereas all health care is included in the registers at the National Board of Health and Welfare, patients must give their approval to be included in the NQRs. The NQR are used in clinical settings, management and research to improve delivery of health care [50].

Sweden has unique personal identification numbers that every person residing in Sweden receives as an identifier [51]. Every person keeps the same number throughout life. The number is used for communication between government authorities, private companies, other organizations, and citizens. It could also be used for research purposes as it allows linking of data from different registers to one specific individual.

All health care provided by Region Skåne and by private care providers under contract with Region Skåne is recorded in Skåne Health care register (SHR). This register includes data on all delivered care within Region Skåne since 1998 on an individual level. Each health care contact generates data entries by the health care provider that are transferred to this central database. Thus, the SHR contains data from visits to all primary care, outpatient specialized care and hospital care. Each record includes ICD-10 (International Statistical Classification of Diseases and Related Health Problems, 10th Revision) [1] diagnostic codes, treatment codes, date of visit, type of care, type of health care professionals. The register is connected to the reimbursement in Region Skåne and the coverage is therefore generally very high [52].

Whereas health care is provided on the regional level, the responsibility for social service and support lies on Sweden’s 290 municipalities. This includes service and support for people with permanent functional impairments according the Act concerning Support and Service for Persons with Certain Functional Impairments (the LSS act) [47]. This includes, for example, daytime activities, residential care, and relief services for next of kin. The goal of the LSS act is that people with functional impairments should be able to participate and be equal members in society and to live an ordinary life as others. The LSS act defines three broad groups of functional impairment, whereof one is those that have ID and/or ASD (Autism Spectrum Disorder) that have been present since birth or early years. The group affiliation and all provided support is recorded in the national LSS register, which is maintained by the Swedish National Board of Health and Welfare. Thus, it is possible to use the LSS register to separate people with ID and/or ASD from those with other kinds of functional impairments and those with intellectual functional impairment acquired in adult life, for example, caused by traumatic brain injury. However, it is not possible to make distinctions within groups, i.e., the register does not comprise information to separate people with ID from those with ASD. It is mandatory for all municipalities to submit data to the LSS register and The National Board of Health and Welfare retrieves data annually.

IDcare is a register study, based on regional and national registers. All people living in Skåne on January 1st, 2014, were included. From these, we created three cohorts, the ID cohort (i.e., people with ID), the ID family cohort (people living in the same household as a person in the ID cohort), and the gPop cohort (general population). As this is a study based entirely on national- and regional registries, participants did not need to be recruited. However, we provided easy-to-read information about the study on a Lund University website (https://www.lupop.lu.se/idcare), offering the possibility to opt out.

Through data from the Swedish total population register people living in Skåne on January 1st, 2014, were identified. The register contains information provided by the Swedish Tax Authority and comprise the civil registration of vital events such as deaths, marriage, and change of address. It comprises all people residing in Sweden and is maintained by Statistics Sweden.

People were included in the ID cohort in two different ways. People that during the study period (2014–2020) had at least one diagnosis of ID (ICD-10 codes F70-F79) or Down syndrome (DS; Q90) in the SHR and/or had support and/or services according to the LSS act and belonged to the ID/ASD group were included in the ID cohort. People living in the same family and/or household as someone in the ID cohort constituted the ID family cohort. The remaining comprised the general population cohort (gPop).

We used various registers at Statistics Sweden to collect data on sociodemographic variables. Due to delays in registration of data, there is some variation in the years of data collection. We collected data about age, gender and place of living 2014–2021. We also obtained data concerning family/household, incomes (for the individual, the family, and the household; 2014–2020), employment status in November each year (2014–2019), and highest level of education (2014–2020).

Data concerning support and service for people with certain functional impairments according to the LSS act [47] was collected from the LSS register for 2007–2020.

SHR for 2014–2021. These included information on date, what kind of contact (e.g., physical visit or telephone call), whether they were planned or not, what kind of specialty and professional were involved, primary diagnosis and up to seven secondary diagnoses. As many variables in the crude data sets had high levels of detail we chose to aggregate to higher levels in our analyses. For example, we aggregated all types of physicians. This means that for example childcare centre physician, maternity care centre physician, pre-registration house officer, senior house office, gynaecologist and teams that included a physician were merged into the higher level “physician”. Similarly, “nurse” included for example childcare centre nurse, maternity care centre nurse, COPD nurse, diabetic nurse, district nurse, hypertension nurse, and teams without physicians. Furthermore, we defined contacts as either physical (new visit, revisit), or non-physical (contact through mail, email, digital meeting online etc.). As the crude data still contain the detailed variables, these could still be used for specific research questions.

Information on the date and place of death, the age at death, and the cause of death registered according to ICD-10 was collected from the Swedish Cause of death register. This register is maintained by the Swedish National Board of Health and Welfare and contains data on all deaths in Sweden.

Three additional registers were used to collect COVID-19-related data for the pandemic period (2020–2021).

The Swedish National Vaccination Register is a health data register maintained by the Public Health Agency of Sweden and used to assess effects of national vaccine programs. Since January 1st, 2021, it is mandatory for health care providers to report all vaccinations for COVID-19 to the register. From this register, we collected data regarding COVID-19 vaccinations, including the date of vaccination, and the type and batch number of the vaccine.

The Swedish Intensive Care Unit (ICU) register (SIR) is an NQR. We collected data from care episodes in the ICU regarding vital parameters and lab samples recorded on admission to the ICU, daily Sequential Organ Failure Assessment scores, a selection of pre-existing diagnoses, and care episode specific diagnoses. Moreover, a burden of care scoring consisting of eleven different parameters and measures the patient-specific workload for nursing staff, recorded three times per 24 h was collected.

The Swedish register of palliative care is an NQR that includes data about the care of people during their last week of life. Data are collected by a questionnaire which is answered by health care staff after the person has died. The collected data from this register comprised data about place of death, prescription of drugs against end-of-life symptoms, assessment of symptoms and information provided to the caretaker and next of kin.

For the first part of the project, a request was sent to Statistics Sweden to create a study cohort with pseudonymized data. Requests were also sent to National Board of Health and Welfare and Region Skåne to disclose socioeconomic- and health care utilization data respectively. Statistics Sweden coordinated the pseudonymization, merging, and delivery of the data using the personal identification number for every person in the study population. Moreover, family- and household-related data were linked to unique family and household identification numbers, respectively. When the project was expanded to include the years for the COVID-19 pandemic and the registers related to intensive care, vaccination and palliative care, the register holders waited for the additional ethical approval and requests concerning the remaining registers before delivering the data. Statistics Sweden will keep a key for the pseudonymization until 2025, thus enabling further addition of data.