I see it now — “anti-vaxxer” in the form of an adjective.
But “an” before any adjective doesn’t necessarily work.
An "an" would signalize* that (in that case) "anti-vaxxer" was a noun.
Re. "necessarily:" There is not always a perfect way to formulate a sentence (i.e., achieve grammatical perfection), and also remove all ambiguity, and also provide loads of info - one must sometimes make a compromise.
Let's also bear in mind that slightly different "rules" apply to / allowances should be made in the case of headlines.
After 40 years here in Germany, translating such texts into / out of German and Russian, one comes to appreciate such nuances. (Notice how I avoided a "Dangling Participle" there?)
Regards,
*Note that I don't say, "would necessarily denote."