Results: Hospital cohort: significantly higher prevalence of probable delirium in the frail sample, with no difference in fever or cough. Community-based cohort: significantly higher prevalence of possible delirium in frailer, older adults, and fatigue and shortness of breath.
Conclusions: This is the first study demonstrating higher prevalence of probable delirium as a COVID-19 symptom in older adults with frailty compared to other older adults. This emphasises need for systematic frailty assessment and screening for delirium in acutely ill older patients in hospital and community settings. Clinicians should suspect in frail adults with delirium. COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), poses a significant threat to the older population at increased risk from infection, hospitalisation and death [1, 2] . In a study from Wuhan, China, over 70% of 339 patients over 60 years of age with COVID-19 were classified as severe or critical, with a case fatality rate of 19% [3] . In Italy, the highest case fatality rate was observed among older, male, multi-comorbid patients [4] and in an American study, adults >65 years old represented 80% of fatalities [5] . This vulnerability might partially be explained by a greater number of chronic diseases [6] and immunosenescence [7, 8] . It is imperative to rapidly identify and contain infection within this group.
Prevalent symptoms of confirmed COVID-19 infection from a variety of international settings are summarised in Table 1 . It is well-documented that presenting symptoms vary with age for many diseases, including respiratory infections [9, 10, 11] . Symptoms including delirium, weakness, malaise, anorexia, headache, dizziness, falls and functional decline are more common in older people. Less typical presentations of COVID-19 in older adults, including weakness, headache, delirium and absence of fever or cough, have been documented in case reports and small cohort studies (n<60) [12, 13, 14] . In work by our research group using point-of-care testing, hypothermia was noted to be an early clinical sign in a minority of older adults [15] .
Frailty, an increased vulnerability to physiological stressors [16] , confers increased risk of adverse health outcomes including mortality, falls, fractures, hospitalisation, and disability [16, 17] . Frailty is associated with increased likelihood of hospital-acquired complications, including delirium, falls or deconditioning. National Institute for Health and Care Excellence (NICE) UK guidelines recommend that all adult patients over 65 years should be assessed for frailty on hospital admission to guide appropriate management and advanced care planning, using the Clinical Frailty Scale (CFS) [18] . In
a large multi-site study, Hewitt et al. showed that outcomes from COVID-19, including mortality, were better predicted by frailty than age or comorbidity [19] . Interestingly, a pre-print study from a single London trust found no association between frailty and mortality rates after COVID-19 and suggested potentially different pathways to mortality in the two groups [20] . However, despite calls for data on the effects of frailty on COVID-19 presentation and management [21] , a recent review retrieved very few studies: all observational, many in small, highly selected cohorts [22] .
We use point-of-care data from patients admitted to a large UK hospital trust, supported by community-based COVID Symptom Study mobile application ("app") data, to assess how frailty affects presentation of confirmed COVID-19 infection in older adults.
This study uses two cohorts.
(i) Hospital cohort: [27] adapted for large-scale app usage, to assess level of frailty (Appendix 1).
PRISMA-7 has been validated for use in older adults [28] and has good validity compared to the CFS [29] . The app was upgraded in April 2020 to allow family members or caregivers to submit data by-proxy, with consent. This has increased numbers of reporting older, frailer participants.
Consent to data analysis and sharing was obtained when individuals enrolled in the app. test. This was filtered to include those aged 65 years and above. All longitudinal entries were condensed per person to form a binary metric recording whether the person had experienced each of the 14 symptoms (below) during the study period. As such, there were no missing data points in the aggregated data.
Probable delirium was diagnosed as a clinically documented delirium diagnosis on admission in the hospitalised cohort; use of the 4-AT screening tool for delirium detection is encouraged but not mandated. Possible delirium in the community-based cohort was classified as "Yes" to question: "Do you have any of the following symptoms:
confusion, disorientation or drowsiness?
Descriptive statistics report demographic and key clinical characteristics of the study populations: overall and according to frailty. Frailty was classified as a binary variable: CFS≥ 5 = frail; CFS<5 = not frail for the hospital cohort; PRISMA7≥3 = frail; PRISMA7<3 = not frail in the community-based cohort. A binary temperature classification was used in the hospitalised cohort, based on a previous large observational cohort study [30] : ≥37.5°C = fever; 37.5 < afebrile.
Continuous variables were expressed as mean +/-standard deviation, and categorical variables as frequencies by absolute value and percentages (%). Differences in proportions and means of covariates between subjects with and without frailty were assessed using Wilcoxon"s test for continuous variables and Fisher"s exact test for categorical variables. Missing data were excluded from analysis.
Following age-matching by frailty status in the hospitalised cohort (n=210), multivariable logistic regression analysis was performed to assess whether frailty predicts symptoms of confirmed COVID-19 infection on hospital admission, including fever, cough and probable delirium. BMI and sex were covariates. using the Tidyverse packages.
Hospital cohort:
Baseline characteristics of the unmatched hospital study cohort are summarised in Appendix 2. Mean age of 322 individuals with confirmed COVID-19 RT-PCR was 78.58 years (standard deviation (SD) +/-7.93); 154 (48%) were females. 165 (51%) patients were classified as frail. 12 patients were excluded from final analysis due to missing clinical frailty score.
After age-matching, difference in mean age between the frail (77.92 +/-SD 6.64 years) and non-frail group (77.85 +/-SD 7.04 years) was non-significant (Wilcoxon"s p>0.05).
The proportion of females was 39% in both groups. BMI differences between the frail and non-frail groups were non-significant (Figure 1 a-c and Table 2 ). Comorbidities of the matched study cohort according to frailty status are summarised in Table 2 . There
was a higher prevalence of respiratory disease and dementia in the frail compared to non-frail group, but no significant differences in cardiovascular disease, diabetes, psychiatric disease or cancer (Table 2) .
After age-matching, delirium was reported in 40 (38%) of frail and 13 (12%) of non-frail patients with COVID-19 ( )). Baseline characteristics of the age-matched cohort are summarised in Figure 1 (d-f) and Table 2 . Comorbidities are summarised in Table 2 .
After age-matching, frailty was found to significantly predict delirium (P-value 0.038; OR (95% CI) = 2.29 (1.33, 4.00)), after FDR correction for multiple testing ( 1.19, 3.39) ). There were no significant differences between frail and not frail for the other 11 symptoms analysed (Table 3 and Appendix 3). In the age-matched participants reporting delirium, 28 (33%) did not report fever or cough. 37
10 (44%) reported both delirium and fever and 35(42%) reported both delirium and cough.
Note this may not represent concurrent symptoms.
This study demonstrates that prevalence of probable delirium is significantly higher in frail compared to non-frail older adults with COVID-19, highlighting both that a frailty resulting from frailty is imperative to prompt rapid diagnosis, isolation and contacttracing, and to guide assessment and treatment. Our findings alert clinicians to the need to identify frailty and look for atypical COVID-19 presentations. We also identified selfreported fatigue and shortness of breath as more common amongst adults with frailtyconsistent with a decrease in physiological reserve.
Early identification of frailty facilitates a differential, targeted clinical approach, focusing not only on "typical" COVID-19 symptoms (e.g. fever and cough) but also on symptoms, such as delirium, demonstrated here to be more common in frail older adults. 
This is the first study reporting that frail patients with COVID-19 present with a significantly higher prevalence of probable delirium than non-frail patients, using hospital and community-based cohorts. Our data emphasises the need for systematic assessment 