 In Period 3, Post-treatment Challenge will take place 19-25 weeks after the first administration in the Treatment Period and will be timed to be outside the Ragweed season. However in the event that the Ragweed season lasts beyond 26th October 2009, the PTC will take place 22-28 weeks after the first administration in the Treatment Period. Assessments performed will be identical to those at the Baseline Challenge. Follow-up will be conducted 3-10 days after PTC. 