I was pro-vaccine from the initial indoctrination that we all get.  I was pretty much pro-everything which was marketed as 'science'.  When I started looking into the science behind vaccines I expected that it would re-enforce my pro-vaccine beliefs.  What I found eventually shifted me 180 degrees.

Actually the above is more true of 'global warming'.  I think with vaccines the shift started when I was writings from old-time social engineers about how injections could be used in the future to shape and mold human populations coupled with lots and lots of reports of injury from victims that piqued my interest enough that I started reading and observing the scientific and medical papers.

It is hard not to notice that those arguing pro-global-warming and pro-vaccine lean heavily on appeal to authority and actively discourage any deeper understanding of the fundamental principles of science.  OTOH, people who don't tow the mainstream line on these topics often bend over backwards to teach basic things like what the n=0.05 in a statistical calculation.  The apperent reason is that when real science and real understanding wins, they win.