It's pretty screwed up this doesn't get more attention. I don't know how many times I've seen comments "explaining" how "vaccine" has always meant the second definition.

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.

Merriam Webster's old definition contradicted other dictionaries and actual usage. Probably you heard about diphtheria and tetanus vaccines before 2020. Those are prepared from toxins. And Merriam Webster's virus definition said viruses aren't organisms. So their old vaccine definition excluded anything made from a virus. Other popular dictionaries had better definitions and didn't change them.

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.

I’d suggest you’re talking about “a definition” not “the definition”. Merriam-Webster is just a dictionary, not the final authority on the meaning of words.

“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.