A Study to Learn How Well Dupilumab Works in Adult and Adolescent Participants With Eosinophilic Gastritis With or Without Eosinophilic Duodenitis and the Side Effects it May Have

This study has 2 parts and a 12-week (about 3 months) Follow-up Period for all participants

• Part A is an open-label treatment period lasting up to 24 weeks (up to 6 months). "Open-label" means that you and the study doctors and the staff staff will know that you are taking the study drug
• Part C is an open-label extended treatment period lasting up to 28 weeks (about 7 months). "Extended Treatment Period" means that you will take the study drug for an additional 28 weeks after finishing Part A if you are eligible to take part in this part of the study

Key Inclusion Criteria:

• Adolescent participants will only be enrolled at study sites in countries/regions as permitted by local regulatory authorities and ethic committees (ECs)
• Documented endoscopic biopsy supporting a pathologic diagnosis of Eosinophilic gastritis (EoG) at least 3 months prior to screening
• Screening endoscopic biopsies with a demonstration of eosinophilic infiltration for a diagnosis of EoG, as defined in the protocol
• Completed at least 11 of 14 days of EoG/EoD-SQ eDiary data entry in the 2 weeks prior to the baseline visit
• History (by participant report) of at least 2 episodes of EoG (with or without EoD) symptoms per week in 8 weeks before screening, as defined in the protocol
• An average TSS of ≥ 20 calculated using data collected via the EoG/EoD-SQ eDiary per week for the 2 weeks prior to baseline. An average severity score of ≥4 (on a scale of 0-10) per week for the 2 weeks prior to baseline for at least 2 of the 6 symptoms, as defined in the protocol.

Key Exclusion Criteria:

• Body weight less than 40 kg at screening
• Prior participation in a dupilumab clinical trial, or past or current treatment with dupilumab
• Helicobacter pylori infection
• Any esophageal stricture unable to be passed with a standard, diagnostic, upper endoscope or any critical esophageal stricture that requires dilation at screening
• History of achalasia, Crohn's disease, eosinophilic colitis, ulcerative colitis, celiac disease, and prior gastric or duodenal surgery, as defined in the protocol
• Other causes of gastric and, if applicable, duodenal eosinophilia or the following conditions: eosinophilic granulomatosis with polyangiitis (Churg-Strauss syndrome) or hyper-eosinophilic syndrome
• History of bleeding disorders, esophageal or gastric varices that, in the opinion of the investigator, would put the participant at undue risk for significant complications from an endoscopy procedure
• Initiation or change of a food-elimination diet regimen or re-introduction of a previously eliminated food group in the 4 weeks prior to the screening visit. Participants on a food-elimination diet must remain on the same diet throughout the study
• Planned or anticipated use of any prohibited medications and procedures during the study
• Planned or anticipated major surgical procedure during the study
• Receiving tube feeding or parenteral nutritional at screening

NOTE: Other Protocol Defined Inclusion / Exclusion Criteria Apply