There have been two moments since leaving office that former president Donald Trump has endorsed the coronavirus vaccine for which he so often requests credit.

The first came Feb. 28, when he encouraged the crowd at the Conservative Political Action Conference to “go get your shot.” That advice, though, was nestled in the middle of complaints about how President Biden was handling the vaccine rollout and was part of a lengthy speech that aired on a Saturday afternoon.

Trump’s other encouragement for the vaccine came earlier this month on March 16. During an interview on Fox News, host Maria Bartiromo prompted Trump by asking if he would recommend that viewers get the shot. He would, Trump replied, adding that it was “a safe vaccine” and “something that works.”

Bartiromo’s question came shortly after the country’s top epidemiologist, Anthony S. Fauci, publicly suggested that promotion by Trump would be useful. The reason was obvious: Republicans repeatedly expressed more skepticism about getting vaccinated, and those Republicans were almost certainly more likely to listen to advice from Trump than Fauci or Biden.

The day Fauci offered that thought, CBS News released a poll showing the partisan divide on the vaccine. Nearly three-quarters of Democrats had gotten a vaccine shot or hoped to. A third of Republicans said they weren’t going to.

Since that point, CBS and YouGov conducted an experiment to measure precisely how much of an effect a Trump endorsement would have. They created a short survey in which those who earlier in the month were again asked about their plans, including those who’d expressed hesitancy about getting a vaccine (the “maybe” and “no” respondents).

The crux of the experiment was a prompt that half the respondents were given: Before being asked how they felt about vaccination, they were first asked about the advocacy of Biden (for Democrats and Democrat-leaning independents) or Trump (Republicans/leaners). In other words, half the pool of respondents was primed with information about presidential endorsement.

Among Democrats, those cued with information about Biden’s endorsement were less likely to say that they planned to be vaccinated than the group that hadn’t received that cue (called the control group). But they were far less likely to say that they wouldn’t receive it.

More important were the results among Republicans. While there was no movement among Republicans who’d said in the earlier poll that they weren’t going to get a vaccine, there was movement among those who’d said they would or that they might.

More than half of the control group of Republicans said they were only maybe going to receive a vaccine. But more than half of those primed with Trump’s recommendation said they would. Put another way, those who did hear about Trump in the poll were about 20 points more likely to say they’d get vaccinated than that they might; those who weren’t cued with that information were about 10 points less likely to say they would get vaccinated.

The central issue here is that so few Republicans had heard about Trump’s endorsement. While more than two-thirds of Democrats already knew that Biden was endorsing the vaccine coming into the survey, only about 4-in-10 Republicans said the same.

That’s not really surprising. When Trump gave the vaccine a thumbs-up Feb. 28, the only network to carry his speech was Fox News. The network never again aired that “go get your shot” line. It did appear on cable news again a few weeks later, when mentioned by an MSNBC guest. (It seems unlikely that many Trump voters saw that.)

There was more coverage of Trump saying he would recommend the “safe” vaccine — but not much. The two phrases were mentioned on Fox News and Fox Business six times, including when it first aired.

That’s important because Fox News is by far the most popular news source for conservative Republicans as polling from Pew Research Center shows. Nearly half of that group identified Fox News as their main source for political or election news in November, a far higher percentage than any other source for any other political group.

Trump himself hasn’t done a lot to amplify this message. In part that’s because of his reduced presence in the public conversation following the social-media bans that accompanied his promotion of the protests in Washington on Jan. 6. In part it’s because his occasional phone interviews on Fox and elsewhere have often centered on getting his response to current political happenings, turning them into a sort of Twitter proxy.

The CBS-YouGov data offer that one bit of positive news: that Republicans being aware of Trump’s support for the vaccine would increase the percentage who get it.

If only the cable networks they watched would talk more about it.

correction

The pool of Republicans included in the graph showing the shift has been more accurately described.