Background: The aim of this study is to investigate the impact of COVID-19 related treatment delay on subcutaneous immunotherapy (SCIT) efficacy in patients with allergic rhinitis (AR). Methods: The study was performed in 643 patients with SCIT appointments between February 1 and May 31, 2020. The clinical assessment, performed at baseline (V0) and one year later (V1), included visual analogue scale (VAS); daily symptom score (dSS); daily medication score (dMS); combined symptom and medication scores (CSMS); quality of life (QoL); self-rating anxiety scale (SAS); and self-rating depression scale (SDS) for each patient. Results: At V0, 249 patients were treated on schedule, and 394 were delayed (7 ± 4.68 weeks). Among them, 319 patients (105 on schedule, and 214 delayed) also completed the assessments at V1, with the absence of 25.39% patients due to completion of SCIT, and 25.35% patients were withdrawal. The results of all assessments were within the normal range for all patients at V0 and V1, with the exception of a slightly higher SDS score (56.13) at V0. In the SCIT delayed group. there was a significant positive correlation between the length (weeks) of the delay and SDS score, and this was significantly higher in patients with poor control of nasal symptoms. Conclusions: This study showed the long-term efficacy of SCIT for AR patients, including those who have had to delay normal therapy due to the COVID-19 outbreak. The psychological status of SCIT patients in response to lockdown of hospital services during this critical period should be considered.