Long-term Safety and Efficacy of Efavaleukin Alfa in Participants With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria: * Participant has provided informed consent prior to initiation of any study specific activities/procedures. * Participant has completed the week 52 endoscopy in the phase 2 dose-finding parent study (20170104) and who in the opinion of the investigator may benefit from continued treatment. Exclusion criteria: * Permanent discontinuation of investigational product during the 52- week phase 2 dose finding parent study (20170104) for any reason * Female subjects of reproductive potential must agree not to donate eggs during the study and for 6 weeks after receiving the last dose of investigational product. Disease Related: * Adenoma and dysplasia exclusion criteria: * Any current sporadic adenoma without dysplasia (adenomatous polyps occurring proximal to known areas of colitis) that has not been removed. * Dysplasia occurring in flat mucosa, sporadic adenomas containing dysplasia, and dysplasia-associated lesions or masses will be managed as follows: * Any history or current evidence of high-grade dysplasia. * Any history or current evidence of dysplasia occurring in flat mucosa. * This includes histopathology reporting indefinite for dysplasia, low-grade dysplasia, and high-grade dysplasia. * Any history or current evidence of a nonadenoma like dysplasia associated lesions or masses, with or without evidence of dysplasia. * Any current sporadic adenoma containing dysplasia or any current adenoma-like dysplasia-associated lesions or masses that has not been removed. Other Medical Conditions: * Any malignancy diagnosed during parent Study 20170104, including evidence of cutaneous basal or squamous cell carcinoma or melanoma * Active infection (including chronic, acute, recurrent, opportunistic infections) at the time of eligibility evaluation requiring intravenous (IV) anti-infectives or hospitalization (infections requiring oral and/or topical anti-infective\[s\] for \> 7 days may be allowed in consultation with the Amgen physician). * Required systemic corticosteroid use for any indication other than ulcerative colitis. The only exception is corticosteroids used for the treatment of adrenal insufficiency are allowed. * Plan to receive a live (attenuated) vaccine during the treatment period and up to 6 weeks after the last dose of investigational product in the long term extension study. Prior/Concurrent Clinical Study Experience: * Currently receiving treatment in another investigational device or drug study. Other investigational procedures while participating in this study are excluded. Other Exclusions: * Female participants who are pregnant or breastfeeding or planning to become pregnant or breastfeed during study and for an additional 6 weeks after the last dose of investigational product. * Female participants of childbearing potential unwilling to use protocol specified method of contraception see Appendix 5 (Section 11.5) during treatment and for an additional 6 weeks after the last dose of investigational product. * Participant has known sensitivity to any of the products to be administered during dosing with the exception of participants who exhibited sensitivity in parent Study 20170104 but did not result in treatment discontinuation. * Participant likely to not be available to complete all protocol-required study visits or procedures, and/or to comply with all required study procedures (e.g., Clinical Outcome Assessments) to the best of the participant and investigator's knowledge. * Participant has a history or evidence of any other clinically significant disorder (including laboratory abnormalities), condition, or disease that, in the opinion of the investigator or Amgen physician, if consulted would pose a risk to participant safety, or interfere with the study evaluation, procedures, or completion.