Two 'x's in "anti-vaxxer"

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")

Just to highlight how unusual the [xx] spelling is, the Dictionary of the British English Spelling System (Greg Brooks, 2015) has this to say about the double-letter [xx] in English:

Doubled letter: (does not occur)

Indeed, [x] is like several other consonant letters that are "never or almost never written double": [h, j, q, v, w, & y] (p. 110). Only one exception is noted for [xx]: the brand name Exxon. (For another name, there's also the classic arcade game Zaxxon.) Anti-vaxxer, presumably, is too new or too slang to appear in this spelling dictionary. According to Merriam-Webster editor-at-large Peter Sokolowski, it was first introduced to the dictionary in 2018, and it was first used in 2009 [Snopes]. Understanding its early history may illustrate how the spelling developed.

Vax

Vax before the first usages of anti-vax was mainly an abbreviation used to brand specific vaccines (e.g. M-Vax, YF-VAX). I can find no instances of use outside of the medical community.

Anti-vax to Anti-vax(x)er

Selective search results for anti-vax on Google and JSTOR first yield results between 2005 and 2010. The initial usages occur in medical contexts as shorthand for people opposed to vaccinations:

The final group is those that are zealously anti-vaccination (anti-vax). (Stephen M. Perle and Randy Ferrance, "What's Good for the Goose Is ... Ethics and Vaccinations." *Dynamic Chiropractic, 23.4, 12 February 2005.)

The usage would slowly leak into the larger scientific and skeptic communities. By 2009, a skeptics conference called The Amazing Meeting had a session titled "Anti-Anti-Vax Panel: Steve Novella, Joe Albietz, Harriett Hall, Michael Goudeau" on its schedule (James Randi Educational Foundation). At around the same time, anti-vaxer and anti-vaxxer make appearances in the comment section to neurologist and skeptic Steven Novella's Neurologica blog post titled "A Personal Attack By J.B. Handley" (22 April 2009). Joseph writes:

Anti-vaxer anecdotes rarely make sense.

And cheglabratjoe writes:

Of course, creationism probably isn't going to directly kill anyone, so these antivaxxers are far more dangerous.

The use of both forms, together, within a day suggests that, early on, both forms were in use.

A post later that year from another skeptic group (Australian Skeptics) highlights how skeptics propagated the word: "The 2009 Richard Dawkins Award Goes to Bill Maher, Anti-Vaxer":

Decide for yourself, but we think the only way to fight the scourge of the anti-vaxers is to ensure they do not get a platform in the mainstream media.

Other comment sections from the period also attest to both forms being in use.

Anti-vaxxer

Between 2009 and today, the word has entered general discourse. It is used by both proponents and opponents of child vaccination. For some reason, anti-vaxxer became the preferred spelling, the one that Merriam-Webster selected and most editors went with.

The reason why is arbitrary. Editors and dictionaries usually like consistency, so they tend to choose one form. We could argue various formal explanations for why the form does or doesn't make sense, but it's hard to generalize from such an unusual form.

Finally, at the end of my process I found a Grammarphobia blog post that confirms the gist of this answer, while also including a meditation on why the [xx] spelling caught on over the [cc] spelling:

Why are the spellings “anti-vaxxer” and “anti-vax” more popular than “anti-vaccer” and “anti-vac”? Our guess is that English speakers prefer “xx” and “x” because it’s natural to pronounce them like the “cc” of “vaccine,” while “cc” and “c” could be pronounced like the “c” of “vacuum.”

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.

If you expect English spelling to obey rules, you will often be disappointed.

This Ngram is for American English.

vaxxer