Inclusion Criteria:

• - Written informed consent - Male or female of any race/ethnicity aged 5-17 years - A female subject of childbearing potential must have a negative pregnancy test and be willing to practise appropriate contraceptive methods - A clinical history of HDM allergic asthma - Use of low daily dose of ICS plus LABA or medium/high daily dose of ICS with or without LABA for the control of asthma symptoms - A clinical history of asthma exacerbations in the past two years - One or more of the following within the past 4 weeks prior to randomisation: - Daytime asthma symptoms more than twice/week - Any nocturnal awakening due to asthma - SABA rescue medication needed for treatment of asthma symptoms - Any activity limitation due to asthma - Lung function measured by FEV1 ≥ 70% of predicted value or according to local requirements - Clinical history of HDM AR within the last year prior to randomisation - An average TCRS>0 during the baseline period - Positive specific IgE (defined as ≥class 2, ≥0.70 kU/l) against D.

pteronyssinus and/or D. farinae at screening

• - Positive SPT to D.

pteronyssinus and/or D. farinae at screening

• - Subject willing and able to comply with trial protocol

  Exclusion Criteria:

  - Is sensitised and regularly exposed to animal dander, molds, and/or cockroach or other perennial allergen - Has experienced a life-threatening asthma attack - Within the last month before the randomisation visit (visit 3), has had an occurrence of any clinical deterioration of asthma that resulted in emergency treatment, hospitalisation, or treatment with systemic corticosteroids - Within the last 3 months before the randomisation visit (visit 3) while on high dose ICS treatment, has had an occurrence of any clinical deterioration of asthma that resulted in emergency treatment, hospitalisation, or treatment with systemic corticosteroids - Any SLIT or SCIT treatment with D.

pteronyssinus or D. farinae within the previous 12 months

• - Ongoing treatment with any allergy immunotherapy product - Any clinically relevant condition or chronic disease incl.

malignancy that in the opinion of the investigator would interfere with the trial evaluations or the safety of the subject

• - Has a diagnosis of eosinophilic oesophagitis - A relevant history of systemic allergic reactions - Ongoing treatment with OCS - Treatment with restricted and prohibited concomitant medication - Treatment with an investigational drug within 30 days/5 half-lives of the drug (which ever longest) prior to screening - A history of allergy, hypersensitivity or intolerance to any of the excipients or active substance of the IMP (except D.

pteronyssinus and D. farinae) or to any excipient of the rescue medication provided in this trial - A business or personal relationship with trial staff or sponsor who is directly involved with the conduct of the trial - A history of alcohol or drug abuse - Has previously been randomised into this trial, is participating in this trial at another investigational site or is participating or planning to participate in any other clinical trial during the duration of this trial - Has a history or current evidence of any condition, treatment, laboratory values out of range or other circumstance that in the opinion of the investigator are clinically relevant and might expose the subject to risk by participating in the trial, confound the results of the trial, or interfere with the subject's participation for the full duration of the trial - Has a condition or treatment that increase the risk of the subject developing severe adverse reactions after adrenaline/epinephrine administration - Is unable to or will not comply with the use of adrenaline/epinephrine auto-injectors for countries where this is a regulatory requirement

The trial aims to demonstrate efficacy of the HDM SLIT-tablet versus placebo as add-on treatment in children and adolescents (5-17 years) with HDM allergic asthma based on clinically relevant asthma exacerbations. Additionally, the trial will investigate if the treatment has an effect on asthma symptoms including nightly awakenings due to asthma, asthma medication use, asthma control, lung function, allergic rhinitis and allergic rhinoconjunctivitis. Finally, quality of life (QoL) for subjects and caregivers will be measured. The trial is a randomised, parallel-group, double-blind, placebo-controlled multi-national phase III trial conducted in Europe and North America. The treatment period will be approximately 2 years. Subjects will receive a written asthma action plan.