Allergen immunotherapy (AIT) represents the only treatment option with a disease-modifying effect on the IgE-mediated inflammatory immune response in allergic patients [1][2][3][4][5]. Both routes of administration, sublingual (SLIT) and subcutaneous immunotherapy (SCIT), had demonstrated clinical efficacy and safety [6][7][8]. However, despite initially reported alleviation of allergic complaints, a relevant proportion of patients suffered from recurrent symptoms some years after the cessation of the AIT [9][10][11].Several innovative approaches in AIT, including adjuvant vaccines, have been developed and already moved into the clinical phase [12]. The ultra-short course subcutaneous AIT (POLLINEX R Quattro Plus 1.0 ml; Bencard R Allergy GmbH, Munich, Germany) using tyrosine-adsorbed grass pollen allergoids and monophosphoryl lipid A (MPL R ) as an adjuvant has proven its clinical efficacy and safety in pollen-allergic patients [13][14][15][16]. The treatment consists of four pre-seasonal injections of 300 SU, 800 SU and 2000 SU (twice), according to the summary of product characteristics [17].Recently, we investigated this ultra-short-course subcutaneous AIT as a booster therapy in patients with grass pollen-induced allergic rhinoconjunctivitis who had completed any kind of grass pollen AIT (SLIT or SCIT) at least 5 years prior to study start with initial reduction of symptoms in the first years after the treatment, but then suffered from recurrent symptoms of grass pollen allergy [18]. The study was designed as a prospective, controlled, nonrandomized, patient-preference, non-interventional study (NIS) carried out in 14 study centers in Germany from March to September 2015. After consulting with their allergists, patients (12 years of age or older) chose whether they received one course of ultra-short AIT before the grass pollen season or not. During the season, all patients received (standardized) symptomatic medication according to the EAACI Position Paper [19] free of charge via an Internet pharmacy. 56 (52.3%) patients chose the booster AIT in combination with symptomatic medication if needed during the grass pollen season (booster AIT group) and 51 (47.7%) preferred to take symptomatic medication exclusively (control group). The study consisted of seven visits for booster AIT patients: four treatment visits (V1 to V4) before and three visits during the grass pollen season (V5 at the beginning of the season, V6 during the peak season, and V7 at the end of the season). Patients of the control group had four visits: one (V1) at the study start and three (V5 to V7) during the grass pollen period.The definition of the peak pollen period was based on data from the German Weather Service, Medicine Meteorology (Deutscher Wetterdienst Meteorologie; www.dwd.de). The regional peak pollen period was defined as those consecutive 30 days with the highest pollen concentration, starting from the first detection of moderateto-high pollen counts (stage 2-3) in the nearest pollen count station.