- Remitters (i.e. patients who achieved remission as measured by Clinical Disease Activity Index (CDAI) at both Week 20 and Week 24) will stop their randomized treatment (certolizumab pegol or placebo) and be followed up until Week 52. Remitters who flare up (CDAI ≥11 confirmed at two consecutive visits four weeks apart) between Week 24 and Week 52 will be re-treated with the same dosing regimen of certolizumab pegol (3 administrations of 400mg, given every 2 weeks, followed by 200 mg given every other week) up to and including Week 50.