The purpose of this study is to learn about the safety and effects of the study medicine (called Ritlecitinib) for the possible treatment of hidradenitis suppurativa (HS). HS is a disease causing long lasting painful red skin lumps.

This study is seeking participants who:

• have moderate or severe HS
• have previously received antibiotics for HS that did not help, or could not tolerate antibiotics Participants will be randomly (like a flip of coin) assigned to receive either the study medicine or a placebo (a pill that looks like the study medicine but does not contain any medicine). The study medicine or placebo will be taken by mouth once daily at home. For the first part of the study, participants will receive a loading (starting) dose. For the next part of the study, participants will receive a maintenance (ongoing) dose.

Participants will take part in the study for about 24 weeks (about 6 months). There will be about 10 study clinic visits: a screening visit, Day 1, and then every 1, 2, or 4 weeks until week 16. At each visit, participants will report on their health and have tests such as physical exams, blood and urine tests, vital signs, chest X-rays, ECGs, hearing tests, and questionnaires. Participants will record when they take the study medicine and their HS symptoms every day in an eDiary on a mobile phone.

The experiences of participants receiving the study medicine will be compared to those receiving placebo to help see if the study medicine is safe and effective.

Key Eligibility Criteria:

Inclusion Criteria:

1. Male or female participants ≥18 to ≤75 years of age.
2. Participants with a diagnosis (based on clinical history and physical examination) of moderate to severe HS for at least 6 months prior to Screening Visit and inadequate response to at least 4-week (28 days) treatment with oral antibiotics for the treatment of HS.

Exclusion Criteria:

1. Presence of ≥20 draining fistulae at Screening or BL visit
2. Evidence of other active skin disease or condition at screening
3. Have a known immunodeficiency disorder
4. Having a history of systemic infection requiring hospitalization or parenteral therapy, including history of infection with Mycobacterium TB
5. Specific Viral Infection History (incl. history of herpes zoster, HBV or HCV Infection
6. Current or recent history of clinically significant severe, progressive, or uncontrolled other medical conditions
7. Any medical or psychiatric condition including any active suicidal ideation in the past year or suicidal behavior in the past 5 years

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.