In my experience, anti-vaxxers lean both left and right, often for different reasons. Basically, it all boils down to a distrust of the establishment, whether that's government, or scientists, or whatever. These aren't always the most politically active people, so their leanings are a bit less well defended, and I've found them to often espouse causes on both sides of the spectrum, sometimes on the same subject. For example, I find they're more likely to ask the government to get out of their own lives (right-wing on small-government) but to increase environmental regulation (left-wing on ecology), so when saying whether they're left or right, one must take a balance of all their views to see which way they lean predominantly (abortion, gay marriage, etc.).