However, it's not an appeal to authority when someone is actually an authority on an established subject where it's possible to be an authority on that thing. Which is the case here.
Hope that helps!
Outside of high school debate clubs, it's pretty reasonable to consider the demonstrated expertise of people making claims. I tend to take the opinion of medical researchers about the relationship of HIV and AIDS over the opinions of members of the Foo Fighters and Thabo Mbeki, for example.
The opinion of an expert, in his area of expertise, backed by a career's worth of research, seems a little more likely to be correct than a random on the internet backed by nothing more that "nuh-uh!"
Sure, authority can be a useful heuristic for non-authorities, but is useless in argumentative reasoning, e.g. "X, who is an authority says Y, and so [therefore] Y is true".
But the reason for it being a fallacy is that this sort of argument doesn't provide evidence of any kind. Rather, it makes the assumption that it must be true, because the authority says so.
You got it. You should not have been downvoted above.
Jenny McCarthy and Dave Grohl's opinions on the effectiveness of vaccination are clearly as valid as my doctor's. Weighting my doctor's views above those is clearly a fallacy because debating rules.
I'm not saying that expert opinions don't have merit, I'm saying that expert opinions aren't logically true because they are uttered by experts.
I suggest you look a little more into these "debating rules" (logical fallacies), as they are basically old-timey resoning rules; i.e. popular and intuitive arguments that don't make sense.
P.S. As I recall, the Foo Fighters were mixed up in HIV-denialism, not anti-vaccination.