• Jun Sugihara, Chiaki Iwamura, Tomoya Tateishi, Tadashi Hosoya, Sho Shimada, Kiyoshi Hirahara, Shinsuke Yasuda, and Yasunari Miyazaki.
• Department of Respiratory Medicine, Graduate School of Medical and Dental Sciences, Institute of Science Tokyo, Tokyo, Japan.
• Clinics (Sao Paulo). 2025 Jan 28; 80: 100584100584.

BackgroundPost-acute COVID-19 Syndrome (PACS) occurs in some COVID-19 patients long after acute infection and significantly affects patients' health. However, the mechanism by which PACS develops is unknown. Myosin light chain 9 (Myl9), produced by activated platelets, plays a role in immune dysregulation and microthrombi formation during acute COVID-19. However, in the PACS phase, the association between Myl9 and residual symptoms remains unclear, and further investigation is needed.MethodsIn this prospective cohort study, serum Myl9 concentrations were measured in 195 COVID-19 patients during hospitalization and at 3- and 6-month follow-up visits. Gaussian mixture modeling was used to identify groups on the basis of Myl9 levels. Relationships between Myl9 levels and residual symptoms were evaluated. Clinical characteristics influencing Myl9 levels were analyzed via logistic regression.ResultsA total of 304 serum samples from 195 patients were collected. Two distinct groups were identified in the Myl9 distribution with a cutoff of 386 ng/mL by Gaussian mixture modeling in this cohort. The high-Myl9 group presented significant residual respiratory symptoms at 6 months post-infection (p < 0.05). Elevated Myl9 levels at 6 months were correlated with increased neutrophil counts (p < 0.01) and respiratory comorbidities at diagnosis (p < 0.05) according to univariate regression analysis. Multivariate regression analysis confirmed the relationship between the neutrophil count and high Myl9 levels.ConclusionProlonged high Myl9 levels are associated with respiratory symptoms, suggesting the potential involvement of prolonged inflammation or endothelial damage in PACS.Copyright © 2025. Published by Elsevier España, S.L.U.

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