Here are RFK Jr.’s exact words.

On June 27, 2023, Kennedy proclaimed (my emphasis):

Everything from Lyme disease to COVID, and many many other diseases, like RSV, which is now one of the biggest killers of children, came out of, you know, a vaccine lab, and we can go down the whole list of diseases. There’s good evidence that even Spanish flu came from vaccine research, and you know, we don’t know, and we’ll probably never know, but, you know, there are very, very strong articles suggesting that now.

On June 15, 2024, he doubled-down on his remarks, asserting:

There’s good evidence that the Spanish Flu — there’s, you know, not definitive but very very strong evidence the Spanish Flu was vaccine-induced flu, the deaths were vaccine-induced. Originally they said it was a flu but when they’ve gone back and actually they have all the samples from thousands of people, they died from bacteriological pneumonia.

Meanwhile, there’s an old — and thoroughly debunked — conspiracy theory that “a massive military vaccination experiment” of a meningitis vaccine “caused” the 1918 flu pandemic. While a meningitis vaccine trial was conducted at Fort Riley, Kansas, in 1918, it was small and limited in scope. More to the point, meningitis is a bacterium, and the flu is a virus. It is impossible to contract a virus from a bacterial vaccine. And that’s especially true since scientists and medical researchers at the time hadn’t the slightest idea what the 1918 flu consisted of.

That’s beside the point, since both the question and Kennedy’s statements refer to a vaccine for the so-called “Spanish flu.” That, too, is impossible, since a vaccine for this strain of flu wasn’t developed until the late 1930s, and it didn’t become widely available until the 1950s.

He really did claim that. The first successful flu vaccine was created in 1940 (though they were working on a flu vaccine at the time). So what RFK means is that somebody teleported back in time and vaccinated the whole world with a vaccine invented in the future (relative to 1918). That or the worm in his brain has done so much damage that he’s simply incapable of looking at facts. Just what you want for a Health and Human Services Secretary. The sooner he’s replaced by somebody who actually understands science and medicine the better because Captain BrainWorm is going to cost this nation needless deaths with his whacky advice.

The Kansas part is true. The Spanish flu, as far as anyone knows, was first identified at Camp Funston, a military base in Kansas. The first known victim was a cook on the base. The first large-scale press coverage, however, was in newspapers in Spain—and that led to the misleading “Spanish flu” name. Spain was not engaged in the ongoing First World War—countries that were at war suppressed news of the pandemic to conceal weakness.

On the other hand, any claim that vaccines have any connections to a cause of Spanish flu is absurdly wrong, so over-the-top—even for RFK Jr.—that I had to look it up to avoid responding to yet another Quora question making a false claim. He did indeed—on at least two occasions in 2023 and 2024—say that he had read unspecified articles that offered “strong” and “good evidence” that (also unspecified) vaccine research somehow caused the 1918 flu epidemic. This is yet another bizarre conspiracy theory from someone with no medical or scientific training whatsoever.

A typical conspiracy theory hobbyist, he believes he knows stuff about medicine and science that the medical and science communities have somehow missed. It’s as if his mission is to use his ignorance to kill people—which he apparently has already done. Kennedy and his nonprofit exploited a tragic vaccine crisis in Samoa to promote anti-vaccine disinformation that killed children.

As part of Donald Trump’s program to rid the government of personnel with genuine education and expertise, Kennedy is now Secretary of Health and Human Services. That is as bad and dangerous as it sounds.

As many, many people have pointed out in this answer’s comments, a flu vaccine did not even exist until many decades after the Spanish Flu epidemic.