Background/Objectives: Previous studies have demonstrated the safety of pre-seasonal treatment with the mannan-conjugated birch pollen allergoid EP-088-T502. However, the safety of a combined pre- and co-seasonal treatment regimen has not yet been investigated. As climate change is associated with earlier and less predictable onset of birch pollen seasons, planned pre-seasonal allergen immunotherapy may unintentionally overlap with natural pollen exposure. Therefore, evaluation of the safety of treatment administered during the pollen season is of increasing clinical relevance. This study aimed to compare, in a purely descriptive manner, the safety and tolerability of pre-seasonal versus pre- and co-seasonal treatment with EP-088-T502. Methods: In this prospective, open-label, phase III trial (T502-SIT-059) (EudraCT No.: 2022-004082-20), patients (N = 109) who had participated in a preceding pivotal phase III study were offered continuation treatment with active EP-088-T502 (10,000 mTU/mL) across five treatment visits. For the subgroup analysis, all patients who completed their last treatment visit before 9 April 2023 (and, thus, before the start of the birch pollen season in Germany) were assigned to the pre-seasonal group (N = 20). Those who performed the last treatment visit thereafter were assigned to the pre-/co-seasonal group (N = 83). Due to post hoc subgroup allocation and unequal subgroup sizes, all subgroup analyses were purely descriptive. Results: No deaths nor serious adverse events (SAEs) were reported during the study. No epinephrine administration was required. Systemic adverse drug reactions (SADRs, N = 3) occurred in two patients who had previously received placebo. No grade III or IV systemic reactions, according to the German AWMF classification, were observed. Patients receiving pre- and co-seasonal treatment developed smaller wheals (mean diameter) compared with the pre-seasonal group (immediate reactions: 0.6 vs. 0.7 cm; late-phase reactions: 0.3 vs. 0.4 cm at the last treatment visit). This was also reflected in the medians (immediate reactions: 0.2 cm vs. 0.4 cm; late-phase reactions: 0.2 vs. 0 cm at the last treatment visit). Of all AEs that were (possibly) related to EP-088-T502 (N = 89), 74 (83%) occurred at the first three treatment visits (before the birch pollen season). The frequency of AEs appeared descriptively similar between groups for the last two treatment visits. Patients who had received placebo in the previous trial experienced more treatment-related side effects compared to patients who had already received EP-088-T502 in the previous year. Conclusions: These data suggest that EP-088-T502 is safe and well-tolerated, even when administered during the birch pollen season, regardless of prior exposure to EP-088-T502.