Disease burden of viral respiratory infections in people with intellectual disabilities: a scoping review

Abstract

Background

Viral respiratory infections are highly contagious and associated with an increased risk of serious adverse health outcomes in vulnerable populations, including people with intellectual disabilities. However, little is known about the overall disease burden of viral respiratory infections among people with intellectual disabilities.

Aim

This scoping review aims to provide an overview of the existing literature on the disease burden of viral respiratory infections among people with intellectual disabilities.

Methods

Studies published between 1999 and 10 July 2024, were identified through PubMed, MEDLINE, Embase and Web of Science, and included if focused on adults with intellectual disabilities, viral respiratory infections and relevant health outcomes such as infection rate, mortality, hospitalisation, functional impairments or quality of life.

Results

58 articles were included, of which the majority focused on COVID-19 (42/58) or influenza (13/58). Studies varied in their methodology and outcome measures. Evidence on COVID-19 and influenza showed that people with intellectual disabilities experience a higher disease burden from these infections compared with the general population. Particularly disproportionate effects were evident among people with Down syndrome, who had higher mortality estimates and an increased burden of comorbidities associated with severe disease and death.

Conclusion

Although available evidence appeared limited and diverse, this scoping review shows a higher disease burden in people with intellectual disabilities, in particular for COVID-19 and influenza-type viruses, which can be indicative of the impact of other viral respiratory infections as well. This highlights the importance of measures to prevent and control the spread of respiratory pathogens among this group.

WHAT IS ALREADY KNOWN ON THIS TOPIC

• People with intellectual disabilities, particularly those with Down syndrome, are known to be at increased risk of adverse outcomes from respiratory infections. However, the overall impact of viral respiratory infections in adults remains poorly described.

WHAT THIS STUDY ADDS

• Using a broad scoping approach across viral respiratory infections, this review shows that the reported disease burden in adults with intellectual disabilities is mainly described for COVID-19 and influenza, with wide variation in outcomes and limited evidence for other respiratory pathogens.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

• This overview warrants the need for broader pathogen-specific research and more standardised outcome reporting to inform clinical care and pandemic preparedness for adults with intellectual disabilities.

Introduction

Viral respiratory infections (VRI) are among the most common infectious diseases that lead to (severe) illness, additional morbidities or mortality on a global scale.^{1 2} Their highly contagious nature can result in widespread outbreaks, even pandemics, putting the entire population at risk of infection, but particularly vulnerable groups.²

The most common respiratory viral infections include influenza viruses, respiratory syncytial virus (RSV), and coronaviruses, with COVID-19 being one of the most recent.³ These infections often manifest as lower respiratory tract infections such as pneumonia, bronchitis and other respiratory complications, which can worsen pre-existing health conditions and increase the risk of adverse outcomes, including death.⁴ Pneumonia, in particular, is a leading cause of death in individuals with compromised immune systems or pre-existing respiratory conditions.⁵

Adults with intellectual disabilities (ID) represent a particularly vulnerable population, especially those residing in healthcare facilities or receiving home care.^{6 7} People with ID, characterised by limitations in intellectual and adaptive functioning, are estimated to constitute 1%–3% of the global population.⁸ This corresponds to approximately 80–240 million people worldwide who are at increased risk due to multiple factors such as a higher prevalence of comorbidities (eg, respiratory disease, cardiovascular disease, immunodeficiencies), difficulties in early symptom identification and delayed intervention.^{7 9} The risk of exposure to respiratory viruses is being increased by congregate living conditions and frequent caregiver interactions,^{6 7} while impaired immune responses can increase their susceptibility to infections and the severity of the clinical outcomes.¹⁰

While people with ID present specific risks in relation to VRI, relatively little is known about their impact in this vulnerable group that could inform tailored prevention and management. This scoping review therefore aims to map and characterise the breadth of available evidence on the disease burden of VRIs in adults with ID. Specifically, we chart how studies have reported key outcomes such as mortality, hospitalisation rates, attack rate/infection rate, functional impairments (physical and psychological) and quality of life. Where available, we also extract information on demographic and clinical characteristics (eg, genetic syndromes, living environments, comorbidities) to contextualise how disease burden has been described across different subgroups, rather than to quantitatively compare or pool outcomes.

Methods

Study design

As it remains unclear which viral pathogens and to what extent contribute to the disease burden in people with ID, a scoping review was conducted to provide an overview of current evidence. This scoping review was reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR), a reporting guideline specifically developed for scoping reviews.¹¹

Operationalisation of VRI burden

We predefined outcome domains to capture both transmissibility and clinical impact of VRIs. Contagiousness was reflected by infection or attack rates, while virulence was assessed through mortality, hospitalisation and functional impairments. Where available, quality of life measures were also included to reflect broader patient impact.

Patient and public involvement

Patients and/or the public were not involved in this scoping review.

Search strategy

The search strategy was developed in collaboration with a medical research librarian of the Radboud University Medical Center. The search consisted of two concepts: the target population ‘intellectual disabilities’ and ‘viral respiratory infections’. The latter included the terms ‘viruses’, ‘respiratory infections’ as well as ‘lower respiratory tract infections’, such as pneumonia or bronchitis. Searches were performed on 10 July 2024, in PubMed, MEDLINE, Embase and Web of Science (online supplemental methods). Review articles identified through this search strategy were assessed to identify relevant articles not captured by our initial search, but results from these review articles were not included in this investigation.

Selection of studies

Studies were included if:

• They were published between 1999 and 2024.

• The target population was adult (≥18 years) or the adult population was described separately from children.

• Written in English or Dutch.

• They examined people with ID, including studies using a broader conceptualisation, such as learning disabilities or intellectual and developmental disabilities (all referred to as ‘ID’ in this article), or studies focusing on aetiological or syndromic subgroups such as people with Down syndrome (DS).

• They described a VRI and one of the following health consequences: (1) infection measures, expressed as infection rate (percentage that becomes infected with the disease) and attack rate (percentage infected within a limited setting, such as care facilities), (2) mortality rate, (3) hospitalisation rate or the severity of the hospitalisation (eg, intensive care unit (ICU) or oxygen treatment), (4) functional impairments (physical and psychological disturbances) and (5) quality of life, that is, complications of VRIs or symptoms.

Studies were excluded if:

• The target population was not clearly described or where ID cannot be assumed from the described aetiology or syndrome (ie, cerebral palsy, autism).

• They examined the effect of the COVID-19 pandemic on mental and physical well-being without a direct relation to the pathogenic individual effect of the virus.

• They were case studies or case series with fewer than 10 participants.

The title and abstract screening were performed independently by LJvS-O and MM using Rayyan.¹² Full-texts were screened (also by LJvS-O and MM) to assess if the article described a VRI that was associated with a health consequence. Discrepancies in the handling of an article were discussed with LJvS-O, MM and MC until a consensus was reached.

Data charting

Summaries of each article were documented, noting the author, year of publication, study design, country, aim, virus, number of people, if a comparison group was present or not, ID definition and severity, syndrome, data period, setting category (eg, primary or secondary care), living environment (eg, independent or residential care), comorbidities, age mean/median and age range. All details, including relevant health consequences that were associated with VRI, were noted. Data from studies with participants with DS were charted by MM, and all other studies by LJvS-O. All extracted data were subsequently checked for completeness and accuracy by LJvS-O. Results from studies that exclusively contained people with DS are reported separately from other studies, to avoid results on DS specifically being generalised to the broader ID population.

Data quality assessment

All included studies were assessed for quality, using the Joanna Briggs Institute Critical Appraisal Checklist for Cohort Studies.¹³ This checklist was adapted to focus on aspects most relevant to included studies, being (1) definition and identification of people with ID; (2) diagnostic procedure of the VRI; (3) identification and addressing of confounders; (4) validity of outcome measures; (5) statistical analyses. Items were scored ‘+Sufficient’, ‘±Moderate’, or ‘−Insufficient’. LJvS-O assessed the studies and later reached agreement by discussion with MM. Studies rated as insufficient were retained but interpreted with caution.

Results

The search identified 9032 articles after removing duplicates. After screening titles and abstracts, 85 articles were reviewed in full text. This included four existing review articles. Comparison of these with the other included studies confirmed the adequacy of our search approach, while identifying two additional relevant studies. These focused mainly on causes of death rather than respiratory infections, although influenza was mentioned among the causes. There was discussion on 9/85 articles (10.6%), which was resolved through consensus. This resulted in 58 studies meeting the inclusion criteria (figure 1).

Figure 1. Search results and study selection flow chart.

Study and population characteristics

The study designs included 52 retrospective studies, 2 prospective studies and 4 reviews. Most studies were published after 2020 (53/58; online supplemental figure 1). Studies were from North America (USA: 21; Canada: 2; Mexico: 1) or Europe (UK: 13; other European countries: 9), others were multi-country/mixed (8/58), or from Asia (2/58), Australia (1/58) and South America (1/58). There were no studies from Africa. More than half of the studies included a comparison group without disabilities (33/58), while 21 studies did not include a comparison group. Approximately one-third of these studies matched participants on confounding variables such as age and sex (11/32). The population sizes varied largely depending on the study setting, varying from institutional level to population-based, ranging from 44 to 365 895 people with ID, and from 34 to 61 675 400 people without ID.

Diagnostic coding of ID was primarily based on ICD-10 (16/58), while three studies used other coding systems, namely CCAM (1/58), SNOMED (1/58) and Read codes (1/58). Studies used data from primary care settings (5/58), or hospital and public health records (7/59), or from ID-related services such as residential, long-term or home care (17/58). In eight studies the methods used to identify ID were not described in detail.

Information on aetiology was reported in fewer than half of the studies (25/58) and most often concerned DS, either as the sole study population or as a subgroup within a broader ID population. Only one study focused on another specific genetic condition called Prader-Willi syndrome. Severity of ID was reported in a limited number of studies (11/58). A few studies applied additional inclusion criteria, for instance selecting participants based on having a laboratory-confirmed VRI (12/58), while other studies included only participants who were hospitalised (7/58) or deceased (6/58).

Quality assessment of the original studies (online supplemental table 1) indicated that most were of sufficient quality (37/54), with fewer rated as moderate (15/54) or insufficient (2/54).

Of the 58 included articles, 42 specifically examined the disease impact of COVID-19, of which 28 in people with ID^14–41 and 19 (also) in people with DS^{16 17 28 29 41–54} (online supplemental table 2). The remaining articles concerned influenza viruses^55–66 and viruses from the Paramyxoviridae family.^{66 67}

SARS-CoV-2 (COVID-19)

Infection rate/attack rate

Of the 42 COVID-19 studies, 17/42 reported an attack rate or infection rate, while 8/42 only included participants who were already COVID-19 positive as an inclusion criterion and could not be used to assess infection risk.^14–21 Reported rates in people with ID ranged from <1% to 45%, compared with 0.4% to 24% in non-ID groups. Differences between studies reflected variations in study period, population characteristics and methodological approaches, with the highest rates generally observed in those receiving residential care.^22–27 For people with DS specifically, studies reported mixed findings, with infection risk ranging from lower to similar to higher than in groups without ID.^{28 42–44}

Mortality

All studies reported an increased risk of COVID-19-related mortality among people with ID, although outcomes were expressed using different metrics, including HRs, ORs, standardised mortality rates, risk ratios and case-fatality rates (CFRs). Across all measures, the risk of mortality was higher for people with ID, particularly those with DS (figures 2 and 3 and online supplemental table 3). For example, reported ORs for COVID-19 mortality ranged from 1.15 to 4.65 in people with ID,^{17 21} and from 2.8 to 24.4 in people with DS,^{45 46} both relative to the non-ID comparator groups. People with ID and DS were found to be at increased risk for mortality from younger ages onwards than their non-ID comparators, with deaths occurring at about 11 and 21 years earlier, respectively. In both groups, age emerged as a key factor, leading to earlier vulnerability compared with the general population. In DS, this age effect was pronounced (OR 2.41 per 10 years), and male sex and Alzheimer’s/dementia were also associated with increased risk.^{47 48 68}

Figure 2. Case fatality rates (CFRs) of COVID-19 in people with ID, DS and the comparison group (people without ID or DS) as reported in each study. Where available, 95% CIs are shown. Studies reporting more than one target group (eg, both ID and DS) are included with separate estimates per subgroup. DS, Down syndrome; ID, intellectual disabilities.

Figure 3. Different outcome measures of COVID-19 mortality reported across studies, including HRs, ORs, RRs and SMRs. Outcomes are reported per study and by subgroup, referring either to people with ID or specifically those with DS, with the general population used as the reference group. Effect estimates are either adjusted for confounders (indicated by circles) or unadjusted (indicated by triangles). The HRs from the studies of Nab and Williamson et al refer to wave 1, representing the original SARS-CoV-2 variant.^{1 2} RR estimates from Landes et al²⁹ were calculated for multiple jurisdictions; the values presented correspond to Maryland³ and the District of Columbia.⁴ DS, Down syndrome; ID, intellectual disabilities; RR, risk ratio; SMR, standardised mortality ratio.

Hospitalisation, ICU admission and respiratory support

People with ID and DS had higher hospitalisation rates varying from 8.0% to 16.5% compared with 3.1% to 4.5% for non-ID groups.^{21 28 47} People with DS were typically hospitalised about 20 years earlier than the general population,^{49 50} and hospitalisation risk increased steadily with age (OR 1.75 per 10 years).⁴⁷

In people with ID and DS hospitalised for COVID-19, ICU admission rates ranged from 4.3% to 50%.^{45 47} Some studies showed no difference when compared with the general population,^{15 28 48} while others reported higher rates (eg, ORs 1.12–2.43).^{17 51} A single study showed a higher OR for ICU admission in DS (1.97) compared with people with ID in general (1.12).¹⁷

Respiratory support needs were also generally higher in people with ID and DS than in the general population. For people with ID, one study reported a 34% increased risk of invasive mechanical ventilation (IMV),¹⁵ while another study showed higher non-invasive oxygen support but lower IMV.¹⁴ In people with DS, reported ORs for IMV ranged from 2.30 to 4.80,^{17 51} and for non-invasive support the OR was 1.79,⁵¹ when compared with people without ID.

Length of hospital stay was longer in DS than in matched comparison groups in several studies (eg, 16 vs 7 days),^{42 50} though one study showed no significant difference.⁴⁹

Comorbidities

People with ID who tested positive for COVID-19 had a higher prevalence of comorbidities compared with individuals without disabilities. Respiratory, endocrine and circulatory diseases occurred more frequently in the ID population.²⁰ Severe illness due to COVID-19 in people with ID was associated with chronic lung disease, diabetes mellitus, obesity and epilepsy.^{16 28} In people with DS and COVID-19, chronic lung disease was 3–4 times more prevalent compared with those without disabilities.⁴⁴ In addition, being diagnosed with COVID-19 was in people with DS associated with an increased likelihood of chronic respiratory disease and dementia.⁴³

Several comorbidities in people with ID, including DS, were associated with hospitalisation risk, increasing progressively with the number of comorbidities and mortality.²¹ Heart disease, present in half of the people with ID who died from COVID-19, was associated with a 10.6-fold increased risk of dying from COVID-19,²⁴ and risk of COVID-19 death was two times higher in individuals with ID and heart disease.¹⁶ DS in itself was associated with COVID-19 death (OR 5.6).¹⁶ In this group, frequently observed comorbidities in hospitalised individuals, other than cardiopathy and obesity, included hypothyroidism, type 2 DM, dementia, epilepsy and obstructive sleep apnoea.^{29 45 48 50 51 68} Mortality was specifically associated with dementia, cardiovascular disease, leukaemia and thyroid disorders in people with DS.⁴⁴ This DS comorbidity pattern, including neuropsychiatric disorders, has also been reported during respiratory infections other than COVID-19.^{45 48 50 51}

COVID-19 symptoms and complications

People with ID, including DS, often presented atypical or milder COVID-19 symptoms compared with the general population. Rather than the common symptoms such as fever or cough, people with ID more frequently exhibited signs like runny nose and shortness of breath.¹⁴ In people with DS specifically, several self-reported symptoms were reported less frequently than in those without disabilities. These included sore throat, loss of smell, fatigue, joint or muscle pain, headache, cough, diarrhoea and vomiting.^{47 48 51}

During hospitalisation, several studies report increased risk of severe medical complications among people with ID and particularly those with DS. Compared with the general population, people with ID had slightly lower or similar rates of in-hospital respiratory complications, such as pneumonia and respiratory failure.^{14 15} On the other hand, people with DS were 2.6 times more likely to develop severe illness, with severe complications, including viral pneumonia, acute respiratory distress syndrome, secondary bacterial pneumonia, sepsis, oxygen saturation below 95% and dyspnoea.^{16 47 48 50 51} People with ID did have a higher incidence of sepsis and increased in-hospital mortality rates.¹⁵

After hospitalisation, people with ID were less frequently discharged to their homes and more often transferred to long-term care or skilled nursing facilities.¹⁵ Furthermore, they were at elevated risk of developing post-COVID conditions compared with people without disabilities. These included persistent symptoms such as anxiety and depression, abnormal breathing and chronic fatigue.¹⁸

Influenza

13 of the 58 included articles specifically focused on the disease impact of influenza in people with ID, of which seven on DS in particular (online supplemental table 2). For people with DS, some included studies on influenza reported results for mixed populations (adults and children) and/or used a combined diagnosis influenza/pneumonia (in cases without viral pathogen testing). For reasons of comprehensiveness, we also describe the results of these studies, including numbers regarding DS and viral pneumonia not otherwise specified.

Infection rate/attack rate

There were no data available on the attack rate or infection rate in people with ID other than DS. In children and adults with DS, influenza was less common than in matched controls (2.1% vs 3.3%),⁵⁵ whereas viral pneumonia was about 13 times more frequent.⁵⁶

Mortality

Influenza and pneumonia accounted for a larger share of deaths in people with ID (8.71% vs 2.18% in the general population),⁵⁷ and in people with DS (11.9% vs 2.1%); in the latter group, women had a 20% lower risk than men.⁵⁸ During influenza epidemics, mortality was higher in people with ID than in the general population (15.2% vs 5.4%),⁵⁹ with rates in people with DS showing a greater excess (23.3% vs 0.1%).⁶⁰

Hospitalisation and respiratory support

Hospitalisation rates for influenza and/or pneumonia were higher in people with ID, with 23.1% being hospitalised compared with 11.9% in the general population.⁶¹ Pneumonia was often the primary cause of excess mortality, but many studies did not clarify whether pneumonia was a result of an influenza infection.^{57 61–65} In people with DS, 20.3% of respiratory admissions involved influenza and/or pneumonia, compared with 6.8% in those without disabilities. Symptom frequency and time to hospitalisation were similar to those without DS.⁶⁰ In children and adults with DS, 62% were hospitalised and 18% required intubation, compared with 9% and 3% respectively in people without disabilities.⁶⁰

Paramyxoviridae

Four of the 58 included studies assessed viruses other than COVID-19 or influenza, such as family members of the Paramyxoviridae family, including RSV and human metapneumovirus (hMPV) (online supplemental table 2). Two of the four studies were scoping reviews, primarily focusing on the disease impact of RSV in children with DS.^{10 69}

A study primarily examined respiratory illness in children with DS but included a few observations in adults. Regarding comorbidities, and compared with the younger patients in the study, adults aged 22 years and older being admitted in relation to pneumonia and respiratory infections had lower rates of CHD (52% vs 84%), and higher rates of dementia (71%) and dysphagia (56% vs 49%). Adults over 40 years were particularly vulnerable, with 25%–33% of individuals in the 40+age subgroups requiring ICU admission for respiratory infection or pneumonia, and experiencing longer hospital stays. Respiratory illnesses were a significant cause of healthcare utilisation across all age groups.⁶⁶

A different study investigated an outbreak of hMPV infection among 44 people with ID in Japan.⁶⁷ Although hMPV is a recognised cause of respiratory illness, it is not as extensively studied as other respiratory viruses, particularly in vulnerable populations like people with ID.⁷⁰ The attack rate was 20%, with 9 confirmed and 20 presumed cases. No mortality was reported. Symptoms included fever, coughing, wheezing and rhinorrhoea.⁶⁷

Discussion

This is the first review to provide a comprehensive overview of the disease burden of VRI in adults with ID, including DS. Previous reviews have primarily focused on DS, frequently combining age groups or focusing on children and adolescents.^{10 68 69} This scoping review is therefore the first to focus on the broader adult ID population. Results show a limited evidence base on VRI and the broader adult ID population, with a majority of studies focusing on COVID-19. People with ID showed, with regard to COVID-19, higher infection rates, particularly in congregate care settings. Compared with the general population, they had a higher prevalence of comorbidities and were more likely to present atypical symptoms during infection. These risks were particularly pronounced among people with DS. In addition, both COVID-19 and influenza/pneumonia accounted for increased risks of hospitalisation and mortality in people with ID, in particular for people with DS.

Even though studies indicated an increased disease burden of VRI in adults with ID, the reported outcomes varied, likely due to methodological differences between studies. Mortality was the most reported outcome across all studies. Mortality data are often among the best available and standardised data in registries, unlike many other health indicators which are often inconsistently recorded.⁷¹ However, comparing true mortality risks between studies was difficult due to the wide variety of reported metrics (eg, HRs vs CFRs). In addition, some studies did not control for key confounders such as age, comorbidities or disability level, which diminishes interpretability.⁷² Reported infection and hospitalisation rates among people with ID and those with DS also varied considerably, reflecting differences in study populations, reporting methods and outcome definitions.^{14 15 17 22–28 42–44 47} Establishing methodological standardisation in the ID sector remains challenging, in part because datasets are often fragmented, difficult to link or subject to ethical limitations.^73–75 These limitations suggest the need for improved data linkage and more standardised data analysis in ID research.

Despite variations in study design and methodology, most studies regarding COVID-19 and influenza/pneumonia showed an increased mortality risk in people with ID, which is particularly pronounced in DS, compared with the general population.^{14 30 31 52 53} Their disproportionate vulnerability is likely explained by a combination of immune system alterations associated with DS,¹⁰ a distinct comorbidity profile⁷⁶ and specific living arrangements.^{26 77 78} Comorbidities commonly associated with severe outcomes from viral respiratory diseases, such as respiratory, cardiovascular, endocrine and neurological conditions, are more prevalent in both the ID and DS groups and likely contribute to increased mortality and hospitalisation risk.^{16 20 24 28 29 43–45 47 48 50 51 68} In DS specifically, dementia and hypothyroidism are also reported more frequently among those hospitalised with COVID-19^{45 48 50} but their specific contribution to VRI risk is unknown, partly because their prevalence in this group is already high.²⁹ In the general population, however, both are linked to impaired immune function, increasing susceptibility to viral infections, which may in turn contribute to the development or worsening of these conditions.^79–81 Additional factors, such as more severe disease and higher exposure in congregate care settings,²⁶ may also contribute to the increased mortality risk observed in people with DS. While the attack rate was higher in congregate settings for people with ID,²⁶ broader epidemiological studies focusing on people with DS reported varying infection rates, yet these likely underestimate the true burden of disease among people with ID due to underdiagnosis and incomplete case registration across different living arrangements. Most studies did not correct for living conditions such as long-term care or supported housing.^{28 42–44} If these environmental factors had been included, infection rates might have appeared higher among individuals residing in such environments, where close contact and shared facilities increase exposure risks. Moreover, adherence to infection control measures was frequently inconsistent in institutional settings, reflecting cognitive and communication challenges among residents, as well as structural and organisational constraints within care facilities, which likely further amplified transmission risks.^{82 83} Although the relative contribution of factors such as disease severity, living environment, and adherence to infection control measures remains uncertain, the consistently elevated mortality risk among people with DS highlights the need for targeted attention from healthcare providers and policymakers, particularly regarding preventive care and support in residential settings.

This scoping review showed that research on VRI among people with ID is scarce, particularly regarding pathogens other than COVID-19. The predominance of studies conducted during the COVID-19 pandemic showed a potential bias towards pandemic-related infections. Nevertheless, the available literature consistently suggests poorer outcomes and atypical disease trajectories in this population. Therefore, generic guidelines and public health policies may not fully address the specific vulnerabilities of this population. While living environments such as long-term care facilities may influence exposure and transmission risks,^{26 77} the main priority is to account for the clinical characteristics of people with ID. Developing effective approaches requires insight into the causative pathogens (viral, bacterial or otherwise), and their interplay with underlying genetic or physiological differences.^{10 84} Understanding these mechanisms is essential to design interventions specifically adapted to prevent, enable early identification of and manage infections in people with ID. Nevertheless, future research should also more systematically account for living conditions, to better disentangle environmental and clinical contributors to infection risk and outcomes in this population.

A key strength of this review is the search strategy using broad definitions of ID, including both syndromic and non-syndromic conditions, as well as internationally used terms such as learning disabilities. Yet, despite our broad approach, existing literature showed a lack of detail regarding ID severity, preventing assessment of how disease burden may vary across the severity spectrum; future research should therefore prioritise standardised and transparent reporting of severity to enable more nuanced and comparable insights. Another strength is the broad perspective on VRIs, considering multiple pathogens and a range of clinical outcomes, rather than focusing on a single pathogen or outcome.

However, several limitations should be acknowledged. Although this scoping review aimed to capture evidence on all viral respiratory pathogens in people with ID, most included studies focused on COVID-19 and were published after 2020, suggesting a publication bias driven by public health crises rather than sustained research into health disparities in this population. In addition, studies frequently did not clearly report the type of pathogen underlying pneumonia and combined cases of influenza with cases of unknown origin. Some pragmatic inclusion decisions were therefore necessary. We have chosen to include these studies because, in the results, influenza was consistently reported together with pneumonia, in line with ICD-10 classification where pneumonia is often considered a complication of influenza,⁸⁵ and in line with clinical practice, where pneumonia cases are often of viral origin and where many cases of bacterial pneumonia arise secondary to an initial viral infection. Given the scarcity of influenza-specific data, these studies were retained, including two additional studies identified through reference screening in other literature reviews.^{64 65} In addition, studies combining data from children and adults with DS were still included when adult-specific results could not be fully separated, provided the results were considered relevant to the adult population. While these decisions may have reduced the specificity of some findings, they were deemed necessary given the limited and fragmented nature of the existing evidence base. This underscores the need for more frequent use of viral panels in pneumonia cases, as well as greater adoption of standardised definitions and reporting frameworks in ID research to enhance consistency and comparability across future studies.

This review highlights the limited evidence on VRIs in people with ID and DS, with most studies focusing on COVID-19 and influenza, while other pathogens remain largely understudied. Heterogeneous methodologies, variability in outcome measures, and inconstancy in reporting ID severity limit comparability, yet a disproportionate disease burden and poorer outcomes seem apparent for COVID-19 and the same appears for influenza/pneumonia among people with ID, and with DS in particular. These findings underpin the importance of measures to prevent and control the spread of respiratory pathogens among this group. Standardised, pathogen-specific future research on other VRIs than COVID-19 and influenza, such as RSV and hMPV is needed to better understand disease burden and to inform clinical practice and public health preventive strategies for people with ID.

Data availability statement

Data are available upon reasonable request.