There is no trial to assess the benefits of periodically using biologics during the pollen season in patients with uncontrolled seasonal allergic rhinitis (SAR), who have moderate-to-severe symptoms even after standard-of-care. This trial aimed to evaluate the efficacy and safety of the add-on administration of stapokibart, a humanised monoclonal antibody that targets interleukin-4 receptor alpha, in patients with uncontrolled SAR.In this investigator-initiated, randomised, double-blind, placebo-controlled trial, eligible patients received either stapokibart 600-300 mg weekly (QW), every 2 weeks (Q2W), or placebo QW for 4 weeks. All patients were given mometasone furoate nasal spray and loratadine throughout the trial. The primary endpoint was the mean change from baseline in daily reflective total nasal symptom score (rTNSS) during 2-week treatment. Secondary efficacy outcomes included: the mean change from baseline in daily rTNSS during 4-week treatment; the mean changes and the mean percentage changes from baseline during 2-week and 4-week treatment in 1) daily rTNSS and reflective total ocular symptom score (rTOSS), 2) morning (AM)/evening (PM) rTNSS and rTOSS, 3) AM instantaneous total nasal symptom score (iTNSS) and instantaneous total ocular symptom score (iTOSS), 4) individual nasal and ocular symptoms; the change from baseline in Rhinoconjunctivitis Quality of-Life Questionnaire score during 4-week treatment. Exploratory endpoints included the change of prespecified markers related to type 2 inflammation pre- and post-treatment. Safety, immunogenicity, and pharmacokinetics were also evaluated. This study is registered with www.clinicaltrials.gov (NCT05470647).Between August 17, 2022, and December 28, 2022, 92 patients with uncontrolled SAR were enrolled from 4 centres in China and randomly assigned to receive stapokibart 600-300 mg QW (n = 31), stapokibart 600-300 mg Q2W (n = 30), or placebo QW (n = 31), of whom 86 (93%) completed the study. Both stapokibart Q2W and QW did not significantly improve mean change from baseline in daily rTNSS compared with placebo in 2 weeks. The least-squares (LS) mean differences (97.5% confidence interval [CI]) compared with placebo were -1.0 (-2.3, 0.2) in stapokibart Q2W group (p = 0.065) and -0.2 (-1.5, 1.0) in stapokibart QW group (p = 0.67). For the secondary outcomes, compared with placebo, stapokibart Q2W presented significant improvements in the mean percentage change from baseline in daily rTNSS in 2 weeks (LS mean difference -12.9%, 95% CI -25.3%, -0.4%, p = 0.043), as well as AM iTNSS over 2 weeks (LS mean difference -17.4%, 95% CI -31.0%, -3.8%, p = 0.013) and 4 weeks (LS mean difference -15.4%, 95% CI -29.0%, -1.9%, p = 0.026). Additionally, the nasal congestion score was significantly lower in stapokibart Q2W than placebo during 2-week (LS mean difference -0.4, 95% CI -0.7, -0.1, p = 0.014) and 4-week (LS mean difference -0.4, 95% CI -0.7, -0.04, p = 0.028) treatment. Treatment-emergent adverse events (TEAEs) occurred in 48% (15/31), 33% (10/30), and 61% (19/31) of patients receiving stapokibart QW, Q2W, and placebo, respectively. Most reported TEAEs were sinus bradycardia, hyperlipidaemia, and blood uric acid increased.In this phase 2 trial, both stapokibart regimens had an acceptable safety and tolerability profile but did not significantly improve daily rTNSS in patients with uncontrolled SAR. The efficacy of stapokibart in patients with uncontrolled SAR is being further investigated in ongoing phase 3 trials (clinicaltrials.gov, NCT05908032).Ministry of Science and Technology of the People's Republic of China; Ministry of Education of the People's Republic of China; National Natural Science Foundation of China; Chinese Academy of Medical Sciences.© 2024 The Author(s).