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.