Now that thiomersal has been removed from most US vaccinations (and let's be honest, the flu vaccine at the very most prevents you from being sick a few days, so it's hardly important, and at the very least just sits there and looks pretty, but not as pretty as snowy of course
), the case shifts to aluminum and a few other dangerous chemicals in trace amounts.
With Aluminum, the trace amounts probably aren't enough to do any real damage, and one may find more in processed cheese. The only issue I have with it is the aluminum is injected directly into the blood stream. I know they are supposed to enhance immune response, but there are organic compounds that do the same thing. So basically there's no evidence of danger with aluminum salt, but it's not really necessary to have it in case evidence may come up that something that can be dangerous (aluminum in larger doses can be dangerous) and maybe it's a bad idea to use 90 year old medicine with something so important.