One comment here: the problem in Washington state is NOT a “religious” exemption but rather a “personal philosophy” exemption; you don’t need to be a member of a particular faith to claim it. You just claim vaccines are against your personal philosophy and bam!, exempt.
So as a very first start, removing the “personal philosophy” exemption is a good approach. I’m curious what the statistics are like in states that don’t have personal philosophy exemptions but do have religious exemptions.
Now, given the idiocy of the anti-vaxxers, if we take my suggestion, they will probably form their own church and start claiming religious exemption but at least for now, it’s not the religious exemption that is at issue in Washington state; it’s a personal exemption.