The goal of this clinical trial is to learn if avacopan in combination with short-term (4 weeks) reduced-dose glucocorticoid and rituximab works to treat patients with newly-onset ANCA-associated vasculitis. It will also learn about the long-term safety of avacopan. The main questions it aims to answer are: Is avacopan in combination with short-term reduced-dose glucocorticoid and rituximab as effective as the combination of 20 week reduced-dose glucocorticoid and rituximab in the proportion of the patients achieving remission? Does avacopan lower the relapse rate compared to the 6 monthly rituximab maintenance therapy? What medical problems do participants have when taking long-term avacopan? Participants will: Be treated with avacopan in combination with short-term (until 4 weeks) reduced-dose glucocorticoid and rituximab (at 0 week) or reduced-dose glucocorticoid (until 20 weeks) and rituximab (at 0, 26, 52 and 78 weeks). Be assessed at 0, 4, 8, 16, 26, 52, 78 and 104 weeks regarding disease status (remission/relapse), disease activity by Birmingham Vasculitis Activity Score ver3, disease damage by Vasculitis Damage Index and adverse events. The primary endpoint is remission rates at 26 weeks.