Substantial Amendment SA-01 a) A modification of inclusion criterion No. 6 was necessary, due to a typing error that was overlooked during the initial protocol development and has been corrected. A RAST class equal or higher than 2 is a standard criterion in most allergy studies. The specific house dust mite species to be evaluated was also added for clarity. b) Text addition to clarify that screening assessments could be repeated for rescreened subjects. A description that screening assessments could be repeated during rescreening was added for clarity. c) Text adaptation to clarify how a demonstration of a significant EAR and LAR without rescue medication use was defined. A subject who demonstrated EAR and LAR with rescue medication use at Visit V2 was acceptable. However, if the subject took rescue medication within the first 7 hours after the single step BAP at Visit V4, the profile of FEV1 measurements could not be accepted as a baseline profile. In this latter case, Visit V4 could be repeated once. d) Text modification to allow that the individual provocation dose, which causes a decrease in FEV1 of at least 20%, could be selected by the investigator, if deemed clinically adequate. In addition, further clarifications were added which specify in which cases an adaptation of the initial allergen dose (or extrapolated dose) was required. e) Text addition to describe that if a subject was considered a screening failure at Visit V2, a final safety evaluation during Visit V3 could be conducted. Also, a description of the procedures and measurements to be performed at this final safety visit was added.

Urgent Safety Measure In the summer 2021, the German federal government had made gains in the management of the COVID-19 pandemic by increasing the availability of COVID-19 vaccines to a larger portion of the population.As a result, T-Balance considered the potential impact of COVID-19 vaccination on the management of the ongoing allergic asthma trial and conducted a risk assessment to better understand how this potential impact could be mitigated. The risk assessment was guided by: 1) Historical program data 2) Regulatory and official guidance, 3) Current scientific literature, and 4) Expert medical opinion. The main outcome of the risk assessment was the need to mitigate the possibility that tregalizumab - which works by acting on immune system cells (namely, activation of T regulatory cells) - could affect the efficacy of COVID-19 vaccines (both mRNA and vector-based). Therefore, T-Balance devised an urgent safety measure to protect trial participants against a potential tregalizumab-induced reduction in COVID-19 vaccine efficacy. The premise of the urgent safety measure, which aimed to introduce appropriate intervals between dosing of the Investigational Medicinal Product (IMP) and COVID-19 vaccination, was mainly guided by Pfaar et al., 2021, MHRA Guidance (“Management of COVID-19 vaccination for subjects participating in ongoing non-COVID-19 clinical trials”), STIKO recommendations, and expert opinions.

Substantial Amendment SA-02 Based on the Urgent Safety Measure regarding the safety measures for COVID-19 vaccination, a second substantial amendment was required to include the relevant measures in the documentation. Along with the USM amendment, a substantial CMC amendment was announced that included an extension of the specification limit for the colour of the placebo. In addition, other non-substantial changes (e.g., CMC relevant) that had occurred since the study was approved were included.