Frailty progression following severe infections in adults aged 65 years and above in US and England: two matched cohort studies

BackgroundWe investigated frailty progression after severe infections in adults ([≥]65 years) in the US and England. MethodsWe conducted parallel matched cohort studies using: US Veterans Aging Cohort Study (VACS-National, 2008-2019; median age 74 years; 98% male); and English Clinical Practice Research Datalink (2006-2019; median age 76 years; 45% male). Adults hospitalised primarily for infection (i.e., severe infection) were matched in calendar date order to individuals without severe infection on age, sex, care site, and US only, plus race and ethnicity. We measured frailty using VACS Index 2{middle dot}0 (US) and Electronic Frailty Index (eFI; England). We estimated annual conditional mean frailty differences between adults with versus without severe infection using linear regression adjusting for baseline frailty, demographics, lifestyle factors, infection history, and US only, comorbidities. ResultsMean baseline frailty was higher in those with severe infection than those without (US: 57 v 48; England: 0{middle dot}17 v 0{middle dot}12). At Year 1, adjusted mean frailty was higher among adults with severe infections than those without (US: VACS Index +2{middle dot}0, 95% CI 1{middle dot}9-2{middle dot}0; England: eFI +0{middle dot}005, 95% CI 0{middle dot}005-0{middle dot}006). At Years 2-5, adjusted mean frailty remained higher after severe infection; however, compared to Year 1, differences were smaller in US, and larger in England. Effects varied by infection type (strongest for lower respiratory tract infections, meningoencephalitis (UK only), urinary tract infections, and sepsis). InterpretationIndividuals with severe infections had higher frailty at baseline and follow up than those without. Preventing both frailty and infections is important for improving health in older age. FundingWellcome Research in contextO_ST_ABSEvidence before this studyC_ST_ABSWe searched PubMed (inception to October 27, 2025), for published articles evaluating the association between infections and frailty, with no language restrictions. We used the search terms [(infection OR infectious) AND (frailty OR frail)]. We found fifteen observational studies investigating associations between individual infections (including: HIV, cytomegalovirus, SARS-CoV-2, acute respiratory infection, urinary tract infection, and influenza) and frailty in adults. Frailty measures varied: eight studies used Frieds phenotype index, six used versions of the cumulative deficit index (i.e., Edmonton Frail Scale, FRAIL-NH Scale, Hospital Frailty Risk Score, Clinical Frailty Score, Veterans Affairs Frailty Index, Vulnerable Elders Survey-13), and one study used the Timed Up and Go Test. Results from identified studies were mixed, with nearly half (7/15) reporting a positive association between the infection studied and frailty, and the remaining eight finding no evidence of association. In cross-sectional analyses, HIV, SARS-CoV-2, cytomegalovirus, and urinary tract infection, were each associated with higher mean frailty scores or frailty prevalence. In longitudinal analysis, hospitalisation for acute respiratory infection was followed by higher mean hospital frailty risk scores two years post-discharge. SARS-CoV-2 infection was associated with early onset (i.e., higher hazard) of frailty over three years follow-up. However, other studies found no association between HIV, SARS-CoV-2, acute respiratory infection and influenza, and frailty prevalence, incidence, or transition between frailty states. These mixed findings may reflect methodological differences between the studies, including variation in frailty measures, and study limitations. Frailty exists along a continuum of vulnerability, and progression after infection may be an important outcome, yet current evidence is scarce. It remains unclear whether severe infections or different types of infection, are associated with faster frailty deterioration. Similarly, it is uncertain whether post-infection frailty risk varies by pathogen (bacterial, viral, parasitic, fungal), infection type (sepsis, urinary tract infection, skin and soft tissue infection, meningitis/encephalitis, lower respiratory tract, gastroenteritis), or by age, sex, social deprivation, and pre-existing comorbidities. Added value of this studyOur study compared frailty progression over a five-year period between adults aged [≥]65 years with severe infection (hospitalisation primarily due to infection) versus comparators without severe infection. We found higher baseline frailty at severe infection onset than in matched comparators. We saw evidence of increased frailty progression over time in people following severe infections compared to those without, however, these differences were small. We also saw higher risk of worsening frailty progression in older adults and those with dementia. Further, worsening frailty progression varied by infection type (strongest for lower respiratory tract infections, meningoencephalitis (UK only), urinary tract infections, and sepsis). Implications of all the available evidenceOur findings underscore the importance of both frailty and infection prevention in improving health in older age. Additional studies are required to explore other wider life-course influences on frailty, to guide the development of comprehensive preventive strategies.