It has. To see proof Merriam-Webster changed it, compare https://web.archive.org/web/20191013020123/https://www.merri... and https://web.archive.org/web/20220203031512/https://www.merri...
And to see proof the CDC changed it, compare https://web.archive.org/web/20191111050113/https://www.cdc.g... and https://web.archive.org/web/20220205010915/https://www.cdc.g...
It makes sense that the dictionary had updated its entry - they are supposed to be reflecting current usage, not defining words as an authority. But the government making changes is absolutely Orwellian.
The CDC working definition didn't change. They removed a redundant reference to protection from a page for the public because some people claimed it meant anything under 100% effective isn't a vaccine.
“The definition” would refer to how people actually use the word, and examples of pre-2020 writing (as I gave) where e.g. the vaccine for Marek’s is called a vaccine would suggest merriam-webster’s definition was too limited.
That said, I think the old Merriam-Webster definition still applies to the covid vaccine anyway, so this is a bit of a side conversation. “ produce or artificially increase immunity to a particular disease” an increase of immunity suggests that this definition also covers leaky vaccines.
The definition update seems to be more about, as sibling poster said, the first part of the definition not covering mRNA or toxin based vaccines accurately.
Not sure the definition has been updated, this article from 2015 discusses a similar vaccine behavior: https://www.pbs.org/newshour/science/tthis-chicken-vaccine-m...
“ The reason this is a problem for Marek’s disease is because the vaccine is “leaky.” A leaky vaccine is one that keeps a microbe from doing serious harm to its host, but doesn’t stop the disease from replicating and spreading to another individual. On the other hand, a “perfect” vaccine is one that sets up lifelong immunity that never wanes and blocks both infection and transmission. [. . .] But the results do raise the questions for some human vaccines that are leaky – such as malaria, and other agricultural vaccines, such as the one being used against avian influenza, or bird flu.“
That said, the covid-19 vaccines we have do prevent some infections even of the mutant omicron variant, just not 95% of infections like it did against the original strain of COVID-19. And luckily omicron, while still dangerous and not really “mild”, is milder than the delta variant and the vaccine seems to be able to save a lot of lives.