This study aims to present a series of cases reports of- knee arthritis occurring following COVID-19 RNA vaccination and to examine the potential role of these Spike protein-based RNA vaccines in the development of this arthritis. Although musculoskeletal disorders have been reported in connection with COVID-19 vaccines, post-vaccination arthritis is not yet listed. Given the different and specific mechanisms of Spike protein-based RNA vaccines and viral vector-based DNA vaccines, we report 7 cases that question the role of COVID-19 vaccines in the onset of early or late-onset knee arthritis observed following one or more injections. All patients (5 early onset and 2 late-onset cases) were examined in the same department and underwent a comprehensive assessment to investigate the usual causes of unilateral or bilateral oligoarthritis; none of them had a previous predisposition to rheumatic disease. No inflammatory rheumatic disease was detected, nor did any develop after a follow-up period of at least 24 months. A double-check of medical histories (both prior to and following diagnosis) was carried out by consulting the general medical database for all patients described. Post-vaccination serological monitoring of blood and synovial fluid to detect the presence of anti-spike antibodies has been requested for all patients and a synovial biopsy was performed in four of them. All patients showed improvement following low-dose prednisolone treatment within two months, but in some patients the symptoms persist. Anti-Spike antibody levels were found to be elevated in blood and synovial fluid samples from all patients. Synovial biopsies reveal chronic histiocytic (CD68+) and plasmocytic infiltration (CD3+) accompanied by neovascularisation. The similar timeline, progression and clinical manifestations of this knee arthritis might suggest a pathogenic role for the spike protein and/or an overproduction of anti-spike antibodies following mRNA vaccination.

Objectives: To compare the psychometric properties (inter-rater reliability, agreement, responsiveness, and measurement error) of the Psoriatic Arthritis Ratingen Score (PARS) and modified Sharp-van der Heijde (SVDH) methods using the Belgian Psoriatic Arthritis (BePas) cohort. Methods: Radiographs of hands and feet (baseline T0, follow-ups T1, T2) were independently scored by two blinded readers using SVDH (max 528) and PARS (max 360). Statistical analyses included Intraclass Correlation Coefficient (ICC), Bland-Altman analysis, Standardized Response Mean (SRM), and Smallest Detectable Change (SDC). Results: Both methods detected positive mean change, primarily between T0 and T1. Inter-rater reliability for absolute scores was high for both (ICC >0.865; baseline ICC: PARS 0.906, SVDH 0.888). For cumulative change scores (ΔT2–T0), inter-rater reliability was low and very similar for both methods (ICC=0.483 for SVDH and 0.485 for PARS). Both methods showed moderate responsiveness over the cumulative interval (two-reader average SRM: PARS 0.571, SVDH 0.531). For both methods and both readers, mean cumulative change scores were smaller than the corresponding SDC values (PARS: 2.8–3.7% of maximum score; SVDH: 2.8–4.1%), indicating that average progression fell below the threshold of reliable individual-level detection. Bland-Altman analysis showed slightly narrower absolute limits of agreement for PARS in selected comparisons, whereas SVDH showed narrower relative limits after normalization to the maximum possible score. Conclusions: SVDH and PARS showed broadly comparable psychometric performance for PsA damage assessment. Both methods were robust for cross-sectional scoring but showed important limitations in reliably detecting cumulative change in this low-progression cohort. Neither method demonstrated clear superiority.