Marked Increase in Influenza-Associated Acute Myositis Among Children During the 2024-2025 Season: A Two-Season Retrospective Cohort Study From Riyadh, Saudi Arabia.

Background Influenza-associated acute myositis is well described in children, but most reports count cases without a denominator of infected children. How often influenza is followed by myositis, and whether that frequency shifts between seasons, therefore remains poorly defined. During the 2024-2025 season, pediatricians at our hospital saw an unusual number of children presenting with calf pain and inability to bear weight after laboratory-confirmed influenza. We compared the frequency of acute myositis among influenza-positive children across two matched seasons, and between influenza A and influenza B. Methodology We studied all unique children aged 0-14 years with influenza A or B confirmed by rapid antigen testing or polymerase chain reaction who presented to the pediatric emergency department of King Salman Hospital, Riyadh, between 1 November and 28 February of the 2023-2024 and 2024-2025 seasons, regardless of disposition. Each period spanned 120 matched days. Acute myositis required confirmed influenza in the same illness, acute calf or lower-limb pain, gait disturbance or inability to bear weight, creatine kinase above the laboratory upper reference limit of 308 U/L, and no documented alternative explanation. Analyses used risk ratios with 95% confidence intervals, chi-square tests, and a modified Poisson model with robust standard errors. Results The cohorts comprised 141 children in 2023-2024 and 173 in 2024-2025. Acute myositis occurred in 19 children (13.5%) in the first season and 73 (42.2%) in the second, an absolute difference of 28.7 percentage points (risk ratio 3.13, 95% CI 1.99-4.93; P<0.001). Pooling both seasons, myositis affected 54 of 107 children with influenza B (50.5%) and 38 of 207 with influenza A (18.4%), with a risk ratio of 2.75 (95% CI 1.95-3.87; P<0.001). The frequency rose within influenza A, from eight of 102 children (7.8%) to 30 of 105 (28.6%), and within influenza B, from 11 of 39 (28.2%) to 43 of 68 (63.2%). Adjusted for influenza type, age, and sex, the season effect persisted (adjusted risk ratio 2.73, 95% CI 1.75-4.26). Case characteristics were similar across seasons: median age 7.7 years in both, peak creatine kinase 4,000 versus 3,710 U/L (P=0.50), and length of stay three versus four days (P=0.49). Myositis-associated admissions rose from 19 to 73 and bed-days from 67 to 264. Conclusions Acute myositis was markedly more frequent among influenza-positive children during 2024-2025 than during the matched preceding season. The increase involved both influenza types, with a stronger association for influenza B, while the presentation and biochemical severity of individual cases were unchanged. These findings show an increased local clinical and hospitalisation burden but do not establish increased viral severity, population incidence, or the emergence of a new influenza variant.