Inclusion Criteria:
* Subject has a history of house dust mite (HDM) induced allergic rhinitis with or without conjunctivitis of 1 year or longer in duration at screening 1.
* Subject has positive skin prick test (SPT) to D. pteronyssinus.
* Subject has a serum specific immunoglobulin E (IgE) level to D. pteronyssinus at screening 1 or within the past 12 months (if performed and documented at the clinical unit).
* Subject shows a positive symptomatic reaction to an HDM based on total nasal symptom score (TNSS) during the challenge test at screening 2.
* Subject has a forced expiratory volume in 1 second (FEV1) of 80% of predicted value or greater at screening 1.
* Subject has a body mass index (BMI) range of 18.5 to 35.0 kg/m2, inclusive and weighs at least 50 kg at screening 1.
* A female subject is eligible to participate if the female subject is not pregnant and at least 1 of the following conditions applies:
* Not a woman of childbearing potential (WOCBP), OR
* WOCBP who agrees to follow the contraceptive guidance starting at screening 1 and throughout the initial safety follow-up period.
* Female subject must agree not to breastfeed starting at screening 1 and throughout the initial safety follow-up period.
* Female subject must not donate ova starting at screening 1 and throughout the initial safety follow-up period.
* A male subject with female partner(s) of childbearing potential must agree to use contraception until after completion of the initial safety follow-up period.
* A male subject must not donate sperm until after completion of the initial safety follow-up period.
* Male subject with a pregnant or breastfeeding partner(s) must agree to remain abstinent or use a condom for the duration of the pregnancy or time partner(s) is(are) breastfeeding until after completion of the initial safety follow-up period.
* Subject agrees not to participate in another interventional study while receiving study drug in present study and until after completion of the initial safety follow-up period.
Exclusion Criteria:
* Subject with concomitant allergies to seasonal aeroallergens which are anticipated to be or become active through the completion of the week 18 daily symptom diary.
* Subject with a concomitant allergy to an animal dander who has exposure on a regular basis to the respective animal dander.
* Subject has received immunosuppressive treatment within 3 months prior to screening 1.
* Subject has received previous immunotherapy treatment with any HDM allergen within 3 years prior to screening 1.
* Subject is receiving ongoing treatment with any specific immunotherapy for other allergies or plans to receive during the course of the primary study period.
* Subject who has used systemic (or inhaled) steroid, mast cell stabilizing drug, T-helper cell type 2 (Th2) cytokine inhibitor, thromboxane A2 synthesis inhibitor, thromboxane A2 receptor antagonist, β-blocker, α-adrenergic blockers, ergot alkaloids, angiotensin-converting enzyme inhibitors and/or angiotensin-receptor blockers within 2 months prior to first study drug administration.
* Subject who has used biologics that are immune modulators within 3 months prior to first study drug administration.
* Subject who has received or is planning to receive vaccination of a live vaccine within 28 days prior to the first administration of the study drug and/or subject who has received or is planning to receive vaccination of an inactive vaccine/toxoid within 7 days prior to the first administration of the study drug during the primary study period.
* Subject with mild to severe asthma receiving therapy.
* Subject has a nasal condition that could confound the efficacy or safety assessments.
* Subject who has history of allergic reactions such as anaphylactic shock, exanthema generalized, angioedema or hypotension caused by HDM and/or any medical products (including vaccine) in the past.
* Subject who has immune disorders and/or diseases requiring immunosuppressive drugs.
* Subject who was diagnosed with immunodeficiency in the past.
* Subject who is unable to discontinue antihistamines.
* Subject has received investigational study drug within 28 days or 5 half-lives, whichever is longer, prior to screening 1.
* Subject has a known or suspected intolerance to lactose and/or milk products.
* Subject has used any prescribed or nonprescribed drugs in the 2 weeks prior to first study drug administration, except for occasional use of paracetamol, topical dermatological products, hormonal contraceptives or hormone replacement therapy (HRT), beta-2-agonist for treatment of asthma or rescue medications for rhinitis and for conjunctivitis, or the coronavirus disease 2019 (COVID-19) vaccine within 7 days prior to the first study drug administration.
* Subject has a history of smoking more than 10 cigarettes (or equivalent amount of tobacco) per day within 3 months prior to screening 1.
* Subject has a history of drinking more than 21 units of alcohol per week (\> 14 units of alcohol for female subjects) within 3 months prior to screening 1 or the subject tests positive for alcohol or drugs of abuse
* Subject has had significant blood loss, donated 1 unit (450 mL) of blood or more, or received a transfusion of any blood or blood products within 60 days or donated plasma within 7 days prior to screening 1.
* Other exclusion criteria apply.
* Subject has a positive result for SARS-CoV-2 PCR test prior to the chamber challenge at screening 2 (repeat virus testing performed 4 to 7 days apart with the second PCR test to be performed a maximum of 2 days prior to the chamber challenge at screening 2) or prior to randomization on day 1.
* Subject has clinical signs and symptoms consistent with COVID-19 infection, e.g., fever, dry cough, dyspnea, sore throat, fatigue, muscle or body aches and gastrointestinal symptoms or confirmed infection by appropriate SARS-CoV-2 PCR test within the 4 weeks prior to screening 1.
