America’s cable news networks seem determined to prove that old adage defining insanity as doing the same thing over and over and expecting different results.

Case in point: The 90-minute town hall appearance that News Nation, an upward-scurrying cable news wannabe, granted Robert F. Kennedy Jr. on Wednesday night. Kennedy says he’s running for the Democratic nomination for president.

The program served largely to underscore a lesson that the media world should have absorbed from CNN’s misbegotten May 10 town hall with Donald Trump: Trying to fact-check anyone who pumps out misinformation and disinformation via a verbal fire hose is a fool’s errand.

News Nation apparently didn’t get the message.

The network tried to avoid one of CNN’s most obvious missteps, which was to place Trump in front of an audience of partisan fans, thus allowing him to evade questions from moderator Kaitlin Collins and play to the gallery.

Instead, it placed Kennedy before a small audience in Chicago and took questions remotely from people in Manchester, N.H., and Charleston, S.C. But the fundamental problem remained: Moderator Elizabeth Vargas was ill-equipped to counter Kennedy’s elaborate web of misinformation about vaccines.

That was a problem, because Kennedy’s anti-vaccination stance is a major element of his presidential campaign. As I’ve written, that’s what makes him a public health hazard.

A key question is why News Nation gave Kennedy this platform. (My colleague Stephen Battaglio wondered the same thing.) By any measure, he’s a fringe candidate for the Democratic Party nomination. His predilection for health misinformation is widely established, and his claims have been widely debunked, including publicly by his own brother, sister and niece.

Perhaps News Nation is trying to assume the mantle of Fox News as a dispenser of right-wing twaddle, or (to be more charitable) of CNN as a sober neutral voice. It’s owned by Nexstar Media Group, the largest owner of TV stations in the country, but as a news source it’s struggling for attention. At the moment, it appears to be a refuge for former cable news retreads.

I can say that it’s emulating Fox as a harbor of bad faith, as I learned recently when, misled by its middle-of-the-road veneer and perhaps motivated by vanity, I accepted an invitation to discuss my June 19 column about Kennedy on the air.

I couldn’t understand why the host, Leland Vittert, seemed so aggressively defensive about Kennedy in our interview — until later that evening, I learned from News Nation’s announcement that it had scheduled this town hall. Suffice to say I won’t be accepting another invitation, in the unlikely event that one is proffered.

On Wednesday night, Vargas didn’t manage to steer Kennedy into the vaccination issue until almost an hour into the program.

I’ll skip lightly through the prelude. Vargas pointed out that Kennedy has garnered the support of right-wing characters including Steve Bannon, Tucker Carlson and Alex Jones, and has even been praised by Trump.

Instead of asking the obvious follow-up, which is what Kennedy thought he has in common with that gang, Vargas asked him: “What sort of man do you think Donald Trump is?” This allowed Kennedy to decline making personal aspersions and declare his commitment to “bring people together.” Never mind that Bannon, Carlson, Jones and Trump are all committed to driving Americans apart.

There were audience questions about how Kennedy would bring down inflation (which has been declining steadily through most of this year) and what he would do about immigration, for neither of which he had a coherent policy answer.

Finally, vaccines. Let’s look at Kennedy’s major assertions and place them in context.

At the start of this segment, Kennedy denied that he is “anti-vaccine.” Instead, he said, “Vaccines should be tested like other medicines. … Unfortunately, vaccines are not safety-tested.”

Then he gave the game away — at least, to anyone with knowledge of how vaccines actually are tested: “Of 72 vaccine doses mandated for American children, not one has ever been subjected to a prelicensing placebo-controlled trial.”

Vargas interjected, “Yes, they have.” But she didn’t catch Kennedy’s deceptive sleight of hand.

Here’s the issue. Kennedy defines the only appropriate safety testing as placebo trials. He demands that vaccines be tested against a control product that has no clinical effect, such as a saline injection.

But that’s wrong. Indeed, in many cases it’s unethical, as pseudoscience debunker David Gorski explains in this lengthy post.

The proper procedure for most vaccines, including almost all those on Kennedy’s roster of 72, is to test them against existing therapies — in most cases, already-existing vaccines. The goal is to test whether the new treatments perform better than the old.

The ethics of placebo testing for vaccines is continually discussed in medical circles. In some cases, it means depriving a control group of a known treatment, which is unethical. In some cases, it means continuing to give control subjects a useless injection after the efficacy and safety of the new treatment have been established, which is also unethical.

None of this means that the vaccines given to American children haven’t been safety-tested. They’ve all gone through phased trials mandated by the Food and Drug Administration to determine their safety and gauge their efficacy.

What Kennedy does — and what he slipped past goalie Elizabeth Vargas — is to exploit the layperson’s lack of understanding about how vaccine testing works to deceive the public into thinking that we’re injecting our children with dose after dose of dangerous vaccines. That’s not responsible.

Kennedy’s other mantra, also aired at the town hall, is that vaccines should be tested for their “long-term risks.” It’s unclear how he defines “long-term,” or how he assesses the consequences of waiting. Does he mean one year? Five years? Thirty years? Some diseases take that long after exposure to manifest themselves. Is 30 years an appropriate period to wait?

Kennedy took aim at two specific vaccines — without any pushback from Vargas. The first is the chickenpox vaccine, which became available in 1995 and is mandated for children in every state.

Kennedy cited what he called evidence that the chickenpox vaccine can cause shingles later in life. This conclusion has been widely questioned in clinical studies.

Shingles, a painful and potentially dangerous rash, is caused by the chickenpox virus, which remains dormant for years after exposure to the disease or vaccination, and can erupt later. Recent studies indicate that the risk of shingles is greater for unvaccinated people than vaccinated. In any case, highly effective vaccines against shingles are available and are generally recommended for people over 50.

What Kennedy specifically said was that the chickenpox vaccine “stops chickenpox, but it causes shingles epidemics.” That’s a gross overstatement — there are no signs of shingles “epidemics” resulting from the shot.

He also said the U.S. rules on the vaccine are at odds with those in Europe. “In the U.S., we mandate it for American children,” he said. “In Europe, they don’t.”

That’s inaccurate. The vaccine isn’t required in Britain or France, but is required in Germany, Spain, Italy, Greece and Luxembourg.

Kennedy also raised an issue with the U.S. mandate — replicated in many other countries — for infant inoculation against hepatitis B, a disease that is commonly transmitted through sexual contact or contact with other bodily fluids.

“Why are we giving hepatitis B vaccine to a 1-day-old child?” he asked. The answer isn’t hard to find: It’s because the hepatitis B virus is far more common than was long supposed and can be passed on from an infected mother — who may not know she carries the virus — to her newborn.

Medical authorities concluded in the 1990s that infant inoculation was a key to reducing the incidence of the disease. But as Elena Conis of UC Berkeley observed in 2011, “vaccinating infants against hepatitis B … is a popular target for contemporary vaccine critics, who for the last decade have argued that the virus is a sexually-transmitted infection that poses little or no risk to babies.”

Kennedy is a carrier of that misconception.

There’s much more about Kennedy’s output of cocksure misinformation than can fit in this space. The danger represented by Robert F. Kennedy Jr. is that while speaking deceptively, he comes off as earnest — a skill that Donald Trump hasn’t mastered.

But the lesson remains: There’s no way that even a determined interviewer can fight back against deception and deceit when it’s dispensed by the torrent.

The greater danger is that our news programs, desperate for attention, can’t learn that lesson … or don’t want to learn.