A Study in Young People and Adults to Learn About the Medicine Ritlecitinib for Treatment of Patchy Hair Loss, Known by the Medical Term as Moderate Alopecia Areata

The purpose of this clinical study is to learn about the safety and effects of the study medicine (called ritlecitinib) for the potential treatment of moderate alopecia areata (AA). This study is seeking participants who are * 12 years or older (if permitted by the local IRB/EC and local regulatory health authority) * have AA with patchy hair loss. The current episode of hair loss has lasted for 6 months or longer but 10 years or less * do not have any other diseases or conditions affecting hair loss. Participants will have a 2 in 3 chance of receiving ritlecitinib 50 mg and a 1 in 3 chance of receiving placebo. The placebo looks like the study medicine but does not contain any active ingredients. Participants will not know what you have been assigned to receive. They will take ritlecitinib or placebo once daily by mouth at home for 24 weeks (6 months). After 24 weeks, the assigned treatment may stay the same or be changed to ritlecitinib 50 mg or 100 mg. This change will depend on how participants' alopecia areata responds to the treatment. Participants will receive the newly assigned treatment for another 23 weeks. About 4 weeks after the last dose, there will be a follow-up visit. At this visit, the team will check on your health. Participants will take part in this study for about 57 weeks. During this time, they will have study visits at the study clinic. Some study checks will be done by phone. We will compare the experiences of people receiving ritlecitinib to those of people who do not. This will help us determine if ritlecitinib is safe and effective.