Inclusion Criteria:
* Participant must be able to understand and provide informed consent
* A clinical history of grass pollen-induced allergic rhinoconjunctivitis for at least 2 years, with peak symptoms in May, June, or July
* A clinical history of moderate to severe rhinoconjunctivitis symptoms for at least 2 years, interfering with usual daily activities or with sleep as defined according to the Allergic Rhinitis and Its Impact on Asthma (ARIA) classification of rhinitis
* A clinical history of inadequately controlled rhinoconjunctivitis symptoms, despite treatment with antihistamines and/or nasal corticosteroids during the grass pollen season, for at least 2 years
* Positive skin prick test response at screening, defined as wheal diameter ≥3 mm to Phleum pratense
* Positive specific immunoglobulin E (IgE) at screening, defined as IgE class 2 (e.g., ≥ 0.7 kilounits per liter \[kU/L\]) against Phleum pratense
* A woman of childbearing potential (WOCBP), regardless of birth control history, must:
* have a negative serum pregnancy test at screening,
* not be breast-feeding or lactating, and ---is required to consistently use one of the following highly effective methods of contraception throughout the study:
* hormonal (e.g. oral, transdermal, intravaginal, implant, or injection),
* intrauterine device (IUD) or system (IUS),
* vasectomized partner,
* bilateral tubal occlusion, or
* sexual abstinence.
Exclusion Criteria:
* Inability or unwillingness of the Subject to give written informed consent or to comply with study protocol requirements
* Prebronchodilator forced expiratory volume (FEV1) \<70% of predicted value at either Screening Visit or Baseline (Visit 0) Visit
* A clinical history of asthma requiring regular inhaled corticosteroids for \>4 weeks per year, outside of the grass pollen season
* A clinical history of moderate to severe allergic rhinitis, as defined according to the Allergic Rhinitis and Its Impact on Asthma (ARIA) classification of rhinitis, caused by either:
* An allergen to which the Subject is regularly exposed, or
* Tree pollen during tree pollen season, treated with regular antihistamine or intranasal corticosteroids
* History of emergency visit or hospital admission for asthma in the previous 12 months
* History of chronic obstructive pulmonary disease
* History of recurrent acute sinusitis, defined as 2 episodes per year for the last 2 years, all of which required antibiotic treatment
* History of chronic sinusitis, defined as a sinus symptoms lasting greater than 12 weeks, that includes 2 or more major factors or 1 major factor and 2 minor factors.
* Major factors are defined as:
* Facial pain or pressure,
* Nasal obstruction or blockage,
* Nasal discharge or purulence or discolored postnasal discharge,
* Purulence in nasal cavity, or
* Impaired or loss of smell.
* Minor factors are defined as:
* Headache,
* Fever,
* Halitosis,
* Fatigue,
* Dental pain,
* Cough, and/or
* Ear pain, pressure, or fullness.
* History of systemic disease affecting the immune system, such as autoimmune diseases, immune complex disease or immunodeficiency
* At randomization: Current symptoms of, or treatment for:
* Upper respiratory tract infection,
* Acute sinusitis,
* Acute otitis media, or
* Other relevant infectious process ---Note: 1.) Serous otitis media is not an exclusion criterion and 2.) Participants may be re-evaluated for eligibility after symptoms resolve.
* A past history of any malignant disease in the previous 5 years
* Any tobacco smoking within the last 6 months, or a history of greater than or equal to 10 pack years of cigarette use.
* Any vaping or electronic cigarette use within the last 6 months
* Previous immunotherapy with grass pollen allergen within the previous 5 years
* Previous treatment by dupilumab (Dupixent®)
* Previous Grade 4 anaphylaxis (World Allergy Organization grading criteria), due to any cause
* History of anti-IgE, anti-IL-5, anti-IL-5 receptor, anti-IL-4/IL-13 receptor, or other monoclonal antibody treatment
* Use of tricyclic antidepressants or monoamine oxidase inhibitors
* Ongoing systemic immunosuppressive treatment
* History of intolerance to the study therapy, rescue medications, or their excipients
* For women of childbearing age a positive serum or urine pregnancy test with sensitivity of less than 50 milli-international units per milliliter \[mIU/ml\] within 72 hours before the scheduled start of study therapy
* The use of any investigational drug within 30 days of the Screening Visit
* The presence of any medical condition that the investigator deems incompatible with participation in the trial
* Past or current medical problems or findings from physical examination or laboratory testing that are not listed above, which, in the opinion of the investigator, may pose additional risks from participation in the study, may interfere with the participant's ability to comply with study requirements or may impact the quality or interpretation of the data obtained from the study
* Eosinophilic esophagitis or a diagnosis of any hypereosinophilic syndrome, and/or
* Administration of live attenuated vaccines within four weeks of dupilumab or dupilumab placebo injections, before the first injection and throughout the treatment period.
