It's true enough.  While in real-world numbers, those percentages work out to 12 million Clinton/19.5 million Trump, I suspect that if you added in disgruntled Sanders and Stein supporters, the balance would shift more to the left.  The difference with anti-vax folks on the left is that there was no overt political agenda - just a lack of critical thinking, which social media (where I firmly place the blame) is ideal at exploiting. Case in point - a few years back, when Mars was going to be at the closest point in its orbit with Earth for something like 60,000 years, several of my Facebook friends - all of whom I know to be reliable blue voters - posted the meme that Mars was going to appear the same size in the sky as the goddamn full moon.

Obviously, right-wing anti-vaxxers marry their own severe lack of critical thinking skills with a genuine own-the-libs anger, so while the shape of the belief is the same, the underlying ethos is dramatically different.

And it should be noted, as you say yourself, that the prominent voices promoting the conspiracy were 'very visible Hollywood people' who I think it's safe(r) to assume are left-leaning - and this shit goes nowhere without a bullhorn.  Rewind the clock, take the Jenny McCarthy-ers out of the equation and I'm certain the anti-vax stuff goes back to being fringe.