Medical experts call vaccines one of the most significant achievements ever in public health and polling data shows Coloradans strongly support them, but still a measure to tighten vaccine exemptions for children is facing a heated political fight. A bill seeking to increase Colorado’s low immunization rates by making the process to request nonmedical exemptions harder was killed last year after the Democratic governor and a vocal minority of parents opposed it. Now, a revamped version is set to be heard in a Senate committee on Wednesday, and parents are again expected to fill the statehouse to testify against it. Senate Bill 163 is scheduled to be heard in the Senate Health & Human Services Committee after the Senate adjourns from its 9 a.m.