This is a Phase 1b/2, randomized, double-blind, multi-center study to evaluate the safety, tolerability, and preliminary clinical efficacy of STMC-103H in neonates and infants at risk for developing allergic disease (Type 1 hypersensitivity). Subjects will be enrolled in a three-part sequential approach. Participants in the safety-run portion of the study (Part A1: 1 year to <6 years of age and A2: 1 month to <12 months of age) will receive 28 days of treatment with STMC-103H or placebo, followed by 28 days of follow-up. A Data and Safety Monitoring Committee (DSMC) will review safety data after all patients in each part complete 28 days of therapy prior to enrolling the next part. After A2, Part B will enroll 224 patients for 336 days of treatment with STMC-103H or placebo, followed by 336 days of follow-up. Stool, blood, and optional samples will be collected in Parts A2 and part B. Primary safety endpoints are frequency, type and severity of Adverse Events (AEs) and Serious Adverse Events (SAEs), as well as findings on physical exams, vitals, and safety laboratories. The primary efficacy endpoint is incidence of physician-diagnosed atopic dermatitis at day 336.

Inclusion Criteria:

• All Parts (A1, A2, B)

  1. Subject's parent(s)/legal representative(s) providing consent must be 18 years or older
  2. Biological mother and/or biological father and/or full sibling(s), have a history of asthma, atopic dermatitis, food allergy, or allergic rhinitis as determined by the screening questionnaire
  3. Subject's parent(s)/legal representative(s) (if appropriate according to local laws) is/are willing and able to give informed consent for participation in the study
  4. Subject's parent(s)/legal representative(s) (if appropriate according to local laws) is/are willing and able, in the PI's opinion, to comply with all study requirements

Part A1 Only

Inclusion criteria 1-4 for all parts plus:

5 (A1). Subject is between 1 year and < 6 years old at the time of enrollment

Part A2 Only

Inclusion criteria 1-4 for all parts plus:

5 (A2). Subject is between 28 days and < 12 months of life at the time of enrollment 6 (A2). Subject's parent(s)/legal representative(s) do not plan to give probiotics (including infant formula that contain probiotics) to the subject during the trial

Part B Only

Inclusion criteria 1-4 for all parts plus:

5 (B). Subject is ≤ 14 days of life at the time of enrollment. Sites should make every effort to enroll newborns as soon as possible after birth.

6 (B). Subject has a birthweight ≥ 2.5 kg and ≤ 4.5 kg 7 (B). Subject's parent(s)/legal representative(s) do not plan to give probiotics (including infant formula that contain probiotics) to the subject from the time of birth to the end of the trial.

Exclusion Criteria:

• All Parts (A1, A2, B)

  1. Subject's twin (or higher order multiple) is enrolled in STMC-103H-102
  2. Subject has any congenital abnormalities or condition, significant disease, illness, physical exam finding, or disorder that, in the opinion of the PI, may put the subject at safety risk or is likely to hinder feeding or affect metabolism that may influence the results of the study. (Neonatal hyperbilirubinemia (jaundice), including jaundice that requires phototherapy, should not be considered exclusionary).
  3. Subject is acutely ill or on systemic antibiotics at the time of enrollment
  4. Subject is participating in another interventional clinical study involving investigational medication, formula, probiotic, or prebiotic use within 30 days (or five half-lives, whichever is longer) of this study
  5. Subject has evidence of immune deficiency/immune compromise in the judgment of the investigator

Part B Only

Exclusion Criteria 1-5 for all parts plus:

6 (B). Subject was born at < 35 weeks' gestation 7 (B). Biological maternal medical condition during the pregnancy that, in the opinion of the PI, may put the subject at risk because of participation in the study. (Maternal antibiotics during the time of delivery should not be considered exclusionary.)

Double-blind, placebo-controlled