I agree with you, that it seems strange that those who are "pro-choice" in the matter of abortion are at the same time "pro-force" in the matter of vaccination.
As for abortion pro-life advocates belief that human life starts with conception and so that abortion is harming or rather killing somebody else's body.
In the matter of vaccination it is of course also somebody else's body than that of the parent that is effected, namely his or her child's.
But while abortion in 99.99% of the cases kills the unborn child, the refusal to vaccinate in 99.99% does not. And by now it seems rather clear that the risk of permanent injury to child by vaccination is far higher than the risk of permanent injury due to rather benign illnesses the child would be vaccinated against.