This study is being conducted to evaluate the safety and effectiveness of the drug ritlecitinib, also known as PF 06651600, in adults living with Chronic Spontaneous Urticaria. This condition is commonly known as hives, which are itchy red welts on the skin that appear without an obvious cause. Some participants may also take loratadine, an antihistamine used to manage allergy symptoms, as part of their regular care.

During the study, participants are assigned to receive either ritlecitinib or a placebo. The study is organized in two stages. The first stage lasts for 12 weeks, after which a second stage, known as an extension period, also lasting 12 weeks, begins. All participants in this phase will follow the same treatment process to see how the medicine affects the severity of the hives over time.