A Study to Evaluate the Long Term Safety and Efficacy of Bimekizumab in Subjects With Ankylosing Spondylitis

Inclusion Criteria: * In the opinion of the Investigator, the subject is expected to benefit from participation in an Open Label Extension (OLE) study * Subject completed AS0008 without meeting any withdrawal criteria * Female subjects must be postmenopausal, permanently sterilized or, if of childbearing potential, must be willing to use a highly effective method of contraception * Male subjects with a partner of childbearing potential must be willing to use a condom when sexually active Exclusion Criteria: * Female subjects who plan to become pregnant during the study or within 20 weeks following the last dose of investigational medicinal product (IMP). Male subjects who are planning a partner pregnancy during the study or within 20 weeks following the last dose * Subjects with any current sign or symptom that may indicate a medically significant active infection (except for the common cold) or has had an infection requiring systemic antibiotics within 2 weeks of study entry * Subjects who meet any withdrawal criteria in AS0008. For any subject with an ongoing Serious Adverse Event, or a history of serious infections (including hospitalizations) in the lead-in study, the Medical Monitor must be consulted prior to the subject's entry into AS0009