COVID-19 immune history and post-PRP injection reactions in knee osteoarthritis

Background: Intra-articular platelet-rich plasma (PRP) is a widely used orthobiologic treatment for knee osteoarthritis (OA). Whether COVID-19 vaccination history or prior SARS-CoV-2 infection influences local post-injection reactions has not been studied. Methods: This exploratory retrospective cohort study included 57 patients who underwent 166 intra-articular PRP injection sessions for knee OA (March 2024–January 2026). Two per-patient outcomes were defined: pain rate (proportion of sessions with VAS pain>0) and swelling rate (proportion of sessions with recorded swelling). Patients with a single injection and no follow-up were excluded. Four Spearman correlations between vaccination dose count, prior infection count, and both outcomes were computed with bootstrapped 95% BCa confidence intervals (1,000 samples) and Holm correction applied across the four p values. Subgroup analyses used the Kruskal-Wallis test. The analytical sample comprised 40 complete cases. Results: All quantitative variables except age were non-normally distributed (Shapiro-Wilk p<0.001). After Holm correction, none of the four primary correlations reached statistical significance. The strongest directional signal was an inverse association between prior infection count and swelling rate (ρ=−0.339, Holm-adjusted p=0.132, BCa 95% CI (−0.560, −0.064), n=40). Post-injection pain (VAS>0) occurred in 20 of 72 assessed sessions (27.8%); swelling in 8 of 75 (10.7%). No febrile reactions were documented. Conclusions: Prior SARS-CoV-2 infection count showed a directional inverse association with post-PRP swelling rate that did not survive Holm correction for multiple comparisons. These findings are exploratory and hypothesis-generating only, and do not support clinical recommendations at this stage.