The purpose of this study is to learn about the safety and effects of the study medicine (called ritlecitinib) for the possible treatment of severe alopecia areata. Alopecia areata is a condition that causes hair loss.

This study is seeking participants who have:

• at least 50% scalp hair loss due to alopecia areata.
• received varicella vaccination (2 doses) or have been infected by varicella zoster virus before based on blood test reports.
• history of clinical response failure to alopecia areata treatment (for children in EU/UK only).

All participants in this study will receive either study medicine (ritlecitinib) or placebo. A placebo does not have any medicine in it but looks just like the medicine being studied.

One-third of participants will receive ritlecitinib higher dose, one-third participants will receive ritlecitinib lower dose, and one-third participants will receive placebo.

The study medicine is a capsule that is taken by mouth. It is taken once each day at home.

The study will compare the experiences of participants receiving ritlecitinib to participants receiving placebo. This will help see if ritlecitinib is safe and effective.

Participants will take part in this study for 6 months. During this time, they will have 8 study visits at the study clinic. The study team will also call participants about 8 times over the phone.

Study B7981027 is being conducted to assess efficacy and safety of ritlecitinib in pediatric participants 6 to <12 years of age with severe AA. The primary objective of this study is to evaluate the efficacy of ritlecitinib compared to placebo in pediatric participants with severe AA on regrowth of lost scalp hair. The secondary objectives are to evaluate safety, tolerability, acceptability and palatability of ritlecitinib and to evaluate the effect of ritlecitinib on patient centered outcomes.

This study will have 3 treatment arms, including 2 ritlecitinib dosage levels (higher and lower doses) and 1 placebo arm. The participants will be assessed for study eligibility at the screening visit after informed consent/assent is obtained (as applicable).

Participants will receive study medication for a duration of 24 weeks.

At least 225 participants will be enrolled in the study. At least 30% of total study population will be recruited from Europe.

The efficacy assessments include Severity of Alopecia Tool (SALT), eyebrow and eyelash assessments. Patient reported outcomes including Patient's Global Impression of Change (PGI-C), Alopecia Areata Patient Priority Outcomes (AAPPO), Patient-Reported Outcomes Measurement Information System (PROMIS) Parent Proxy - Anxiety Short Form 8a and Depressive Symptoms Short Form 6a, Behavior Rating Inventory of Executive Function®, Second Edition (BRIEF®2), and modified Children's Dermatology Life Quality Index (CDLQI) will be assessed throughout the study. Pharmacokinetics of ritlecitinib will be evaluated using sparse sampling.

Safety monitoring will be performed to identify and monitor the known and potential risks of ritlecitinib.

Participants completing the 24-week treatment period of the study may have the option to enter the long-term extension (LTE) Study B7981028, if the eligibility criteria are met. Participants who complete the 24-week treatment period of the study but who are ineligible for the LTE study will undergo a 4-week off-treatment follow-up period.

Inclusion Criteria:

• A diagnosis of AA (including alopecia totalis (AT) and alopecia universalis (AU)) with at least 50% scalp hair loss due to AA (ie, SALT score of ≥50) at both screening and baseline visits, without evidence of terminal hair regrowth within the previous 12 months.
• For study participants in the EU/UK only: History of clinical response failure to AA treatment (such as topical, off-label pharmacologic, or hairpiece prosthetics)
• Documented evidence of having received varicella vaccination (2 doses), OR evidence of prior exposure to varicella zoster virus (VZV) based on serological testing (ie, a positive VZV Immunoglobulin G (IgG) antibody (Ab) result) at screening.

Exclusion Criteria:

• Other (non-AA) types of alopecia, including any known congenital cause of AA.
• Pre-existing hearing loss.
• Any present or history of malignancies or lymphoproliferative disorder such as Epstein-Barr virus (EBV) related lymphoproliferative disorder, history of lymphoma, history of leukemia, or signs and symptoms suggestive of current lymphatic or lymphoid disease.
• Clinically significant depression per PROMIS Parent Proxy Short Form - Depressive symptoms (T-score ≥70).
• Any evidence of untreated or inadequately treated active or latent Mycobacterium tuberculosis (TB) infection; history (one or more episodes) of severe or serious cytomegalovirus (CMV) infection, herpes zoster (shingles) or disseminated herpes simplex; infection with hepatitis B virus (HBV) or hepatitis C virus (HCV).
• Vaccination with live attenuated replication-competent vaccine within 6 weeks of first dose of study intervention.

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.

Participants and their caregivers, Investigators and other site staff will be blinded to their assigned study intervention. Sponsor staff will be blinded to participants' assigned study intervention, except for sponsor staff involved in the assignment or distribution of study intervention and the provision noted below.

Sponsor staff who are not directly involved with the conduct of this study will prepare analyses and documentation containing unblinded data while the study is ongoing