I have always found myself impulsively and automatically spelling “anti-vaxxer” with two ‘x’s, and a Google search indicates that most other media sources did the same; however, I can’t think of any other words in English that contain two adjacent ‘x’s, and I also can’t think of a good consistent rule that would explain why it seems that the ‘x’ should be doubled in this case.
The person who reads is a “reader”, not a “readder”. However if I were to make up a word “to zif”, then “the person who zifs” would probably be a “ziffer”. In that case, you need the extra ‘f’ to distinguish the sound from something that rhymes with “lifer”. Maybe the reason for the double-‘x’ is then that “anti-vaxer” might be mispronounced as “anti-vaikser”?
Anyone understand what is going on here? Are there other words that have an unusual but customary consonant doubling like this?
Update: other example: “doxxed”; also possibly related: “savvy” (from Fr. “savoir”)
I don’t think it’s likely that “vaxer” would be misread as sounding like “vakes-er”. All of the normal existing words with similar spelling (such as wax, ax, tax) are pronounced with short a and never double the x before a vowel-initial suffix.
A related previous question: Why do we write “fixing” instead of “fixxing”?
Also related: Should it be “doxxed/doxxing” or “doxed/doxing”?
If anything, I think xx in anti-vaxxer might be related to it being an abbreviation. That is something that it and dox(x)/doxxing have in common. But I don’t know why double xx would be used specifically in abbreviations.