This research study is being conducted to find out if the test medicine, abrocitinib, improves eczema and is safe for children 6 to <12 years of age who have moderate-to-severe eczema. Research study participants who meet the study criteria will be assigned by chance (like the flip of a coin) to receive either abrocitinib test medicine or placebo (pretend medicine that looks just like the test medicine) for 16 weeks. The study will last for about 24 weeks in total.

Different questionnaires will be used to find out how the participant's eczema is doing during the research study. An electronic diary, which looks like a cellular phone, will be used to collect the questionnaire information and other research study-related information. It will also be used to remind participants to take the test medicine each day and to fill out the questionnaires. Participants will also be asked to wear something on their wrist that looks like a children's wristwatch so we can know when they are scratching their skin because of their atopic dermatitis. Laboratory tests, physical examinations and vital signs (height, weight, temperature, blood pressure and heart rate) will be conducted before and during the research study to monitor your child's health. Participants will need to come to the clinic for research study visits and at other times during the research study, the research study doctor or nurse will call the participants to find out the participant is doing.

Inclusion Criteria Children aged 6 to <12 years at the time of informed consent/assent.

• No contraception methods are required for male participants. Refer to Appendix 4 for reproductive criteria for male (Section 10.4.1) and female (Section 10.4.2) participants.

Disease Characteristics:

Participants who meet all of the following AD criteria:

• A documented diagnosis of chronic AD for at least 1 year prior to screening and confirmed at screening and baseline visits according to the Hanifin and Rajka criteria[19]; and
• A diagnosis of moderate-to-severe AD at the baseline visit (must fulfill all of the following criteria: BSA ≥10%, vIGA ≥3, EASI ≥16, and WI-NRS ≥4); and
• Documented history (within 6 months of the screening visit) of inadequate response to treatment with topical medical therapy for AD (eg, TCS and TCI), for at least 4 weeks and are candidates for systemic therapy

Other Inclusion Criteria:

Body weight ≥15 kg

Exclusion Criteria

Participants are excluded from the study if any of the following criteria apply:

Medical Conditions:

Any medical or psychiatric condition including any active suicidal ideation in the past year or suicidal behavior in the past 5 years or laboratory abnormality that may increase the risk of study participation or, in the investigator's judgment, make the participant inappropriate for the study.

If the participant has SDQ total score ≥17, the investigator should exclude them or refer the child to a pediatric MHP to determine if it is safe to participate in the study. A copy or summary of the evaluation should be placed in the site source documents.

Have any of the following medical conditions:

• Infections:

  • Skin infections that require treatment with systemic antimicrobials within 2 weeks prior to Day 1 (Baseline) or have superficial skin infections within 1 week of Day 1.
  • History of systemic infection requiring hospitalization or parenteral antimicrobial therapy or as otherwise judged clinically significant by the investigator within 1 month prior to Day 1.
  • Have a history (single episode) of disseminated herpes zoster or disseminated herpes simplex, or a recurrent localized, dermatomal herpes zoster.
  • Infection with HIV, hepatitis B, and/or hepatitis C
  • Evidence of active TB or inadequately treated latent TB.

• Skin Conditions:

  - Including but not limited to psoriasis, seborrheic dermatitis or lupus on Day 1 that would interfere with evaluation of AD or response to treatment.

• Other Conditions:

  • Documented history of skeletal dysplasia.
  • Documented history of retinal detachment.
  • History of or conditions associated with thrombocytopenia, coagulopathy or platelet dysfunction.
  • Prior history of leukemia, lymphoma, sarcoma or any other malignancy.
  • Immunodeficiency disorder or a first-degree relative with a hereditary immunodeficiency.
  • Any other medical conditions that in the investigator's judgment make the participant inappropriate for the study.

Prior/Concomitant Therapy:

Prior treatment with a systemic JAK inhibitor for AD. Live attenuated vaccination within 6 weeks prior to Day 1 or require vaccination with live attenuated vaccines during treatment or within 6 weeks after the last dose of study intervention.

Concomitant use of strong inhibitors and inducers of CYP2C19 enzymes, strong inducers of CYP2C9 enzymes, P-gp substrates with narrow therapeutic index and sensitive CYP2C19 substrates is not allowed in the study.

Prior/Concurrent Clinical Study Experience:

Previous administration of an investigational drug within 30 days or 5 half lives, whichever is longer, of Day 1.

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.