Participants must be at least 18 years and ≤60 years for males (≤70 years of age for females) at the time of informed consent

Participants must have severe or very severe AA, as determined by all of the following: - Current AA episode of more than 6 months’ duration and hair loss encompassing ≥50% of the scalp, as measured by SALT (AA-IGA of 3 or 4) at screening and baseline - no spontaneous improvement over the past 6 months - current episode of severe or very severe AA of less than 8 years. Note: participants who have severe or very severe AA for ≥8 years may be enrolled if episodes of regrowth, spontaneous or under treatment, have been observed on the affected areas over the past 8 years

Participant must be male or nonpregnant, nonbreastfeeding female participants

Participants must not have primarily “diffuse” type of AA

Participants must not be currently experiencing other forms of alopecia or any other concomitant conditions that would interfere with evaluations of the effect of study medication on AA

Participants must not have been previously treated with an oral Janus kinase (JAK) inhibitor and had an inadequate response (for example, absence of significant terminal hair growth after at least 12 weeks of treatment)