Severe Alopecia Areata

The purpose of this observation study is to learn about the safety and effects of the study medicine (called Litfulo) for the potential treatment of Severe Alopecia Areata.

A multicenter clinical study to evaluate the safety, PK characteristics, immunogenicity characteristics, and PD characteristics of IBI3013 in healthy trial participants and active non-segmental vitiligo trial participants and severe alopecia areata trial participants. The study is divided into 2 parts, with Part 1 involving healthy trial participants lasting up to 24 weeks, and Part 2 involving active non-segmental vitiligo trial participants and severe alopecia areata trial participants lasting up to 48 weeks.

The purpose of this clinical trial is to learn about long-term safety and long-term effects of the study medicine (called ritlecitinib) for the potential treatment of severe alopecia areata, a condition that causes hair loss. This study is seeking participants who have: * previously completed one of Pfizer's pediatric studies for Alopecia Areata (B7981027 or B7981031). * at least 50% scalp hair loss due to alopecia areata (for participants enrolling from the study B7981031). * received varicella vaccination (2 doses) or have been infected by varicella zoster virus before based on blood test reports. All participants in this study will receive the study medicine (ritlecitinib). Participants who received ritlecitinib higher or lower doses in the parent Study B7981027 will continue receiving the same ritlecitinib dose in this trial. Participants who received placebo in the parent Study B7981027 and all participants from parent Study B7981031 will receive either higher or lower dose of ritlecitinib in this trial. The study medicine is a capsule that is taken by mouth. It is taken 1 time each day at home. The study will help see if ritlecitinib is safe and effective. Participants will take part in this study for a duration of up to 3 years (36 months). During this time, they will have 17 study visits at the study clinic. The study team will also call participants once a month over the phone.

The purpose of this study is to evaluate the efficacy and safety of different doses of SYHX1901 tablets in the treatment of severe alopecia areata.