All-cause and cause-specific mortality among people with and without intellectual disabilities during the COVID-19 pandemic in the Netherlands: a population-based cohort study

Background
Although high rates of COVID-19-related deaths have been reported for people with intellectual disabilities during the first 2 years of the pandemic, it is unknown to what extent the pandemic has impacted existing mortality disparities for people with intellectual disabilities. In this study, we linked a Dutch population-based cohort that contained information about intellectual disability statuses with the national mortality registry to analyse both cause-specific and all-cause mortality in people with and without intellectual disabilities, and to make comparisons with pre-pandemic mortality patterns.

Methods
This population-based cohort study used a pre-existing cohort that included the entire Dutch adult population (everyone aged ≥18 years) on Jan 1, 2015, and identified people with presumed intellectual disabilities through data linkage. For all individuals within the cohort who died up to and including Dec 31, 2021, mortality data were obtained from the Dutch mortality register. Therefore, for each individual in the cohort, information was available about demographics (sex and date of birth), indicators of intellectual disability, if any, based on chronic care and (social) services use, and in case of death, the date and underlying cause of death. We compared the first 2 years of the COVID-19 pandemic (2020 and 2021) with the pre-pandemic period (2015–19). The primary outcomes in this study were all-cause and cause-specific mortality. We calculated rates of death and generated hazard ratios (HRs) using Cox regression analysis.

Findings
At the start of follow-up in 2015, 187 149 Dutch adults with indicators of intellectual disability were enrolled and 12·6 million adults from the general population were included. Mortality from COVID-19 was significantly higher in the population with intellectual disabilities than in the general population (HR 4·92, 95% CI 4·58–5·29), with a particularly large disparity at younger ages that declined with increasing age. The overall mortality disparity during the COVID-19 pandemic (HR 3·38, 95% CI 3·29–3·47) was wider than before the pandemic (3·23, 3·17–3·29). For five disease groups (neoplasms; mental, behavioural, and nervous system; circulatory system; external causes; and other natural causes) higher mortality rates were observed in the population with intellectual disabilities during the pandemic than before the pandemic, and the pre-pandemic to during the pandemic difference in mortality rates was greater in the population with intellectual disabilities than in the general population, although relative mortality risks for most other causes remained within similar ranges compared with pre-pandemic years.

Interpretation
The impact of the COVID-19 pandemic on people with intellectual disabilities has been greater than reflected by COVID-19-related deaths alone. Not only was the mortality risk from COVID-19 higher in people with intellectual disabilities than in the general population, but overall mortality disparities were also further exacerbated during the first 2 years of the pandemic. For disability-inclusive future pandemic preparedness this excess mortality risk for people with intellectual disabilities should be addressed.

Funding
Dutch Ministry of Health, Welfare, and Sport and Netherlands Organization for Health Research and Development.