Staff from San Mateo County and Dignity Health manage a mass vaccination clinic for Phase 1A health care workers at the San Mateo County Event Center, Calif. in March 2021. (Photo courtesy County of San Mateo Joint Information Center)

Former President Donald Trump’s endorsement of the COVID-19 vaccine may have spurred some skeptics to get vaccinated, according to a study released Monday by researchers at a quartet of universities.

Researchers at four universities, including Stanford and the University of California, Berkeley, created a 27-second advertisement showcasing Trump’s support for the vaccine and placed it on videos across more than 150,000 YouTube channels in more than 2,000 counties across the country.

The advertisement played between Oct. 14 and Oct. 31, 2021, in 1,083 counties with vaccination rates below 50 percent. Another 1,085 counties did not receive the ad and were included as a control group.

The study analyzed the number of vaccine doses administered in the relevant counties one month before the campaign and one month after and found that 104,036 more shots were administered in counties that received the advertisement than those that didn’t.

That increase, however, was limited to counties in which fewer than 70 percent of voters chose Trump in 2020, as the counties with the most Trump voters showed little movement following the campaign.

According to UC Berkeley’s Haas School of Business, the researchers spent just under $100,000 on the advertising campaign, amounting to about $1 for each new vaccine dose.

“Creating an intervention that effectively costs about $1 per extra vaccine is remarkably cost-effective, and a small fraction of the cost of other interventions,” said Steven Tadelis, a co-author of the study and a professor of business and public policy at the Haas School.

The team behind the study, which also included researchers from the University of North Carolina at Chapel Hill and North Carolina State University, set out to determine a potential method of breaking through the partisan divide among those who remain unvaccinated.

According to data from the Kaiser Family Foundation, three-fifths of the U.S. residents who were unvaccinated in October identified as Republicans, compared to just 17 percent who identified as Democrats.

That gap has persisted even after Trump and former First Lady Melania Trump got vaccinated privately at the White House in January 2021, shortly after the vaccines became available. Trump said in December 2021 that he had also received a booster vaccine dose.

“We felt like there should be a better way to send a message that would resonate with people on the right,” said Brad Larden, the study’s lead author and an economics professor at Stanford University.

The advertisement included clips of a local news anchor in Salt Lake City, Utah, commenting about the Trumps getting vaccinated.

It also included a portion of an interview between Trump and Fox News anchor Maria Bartiromo from March 2021 in which he said “I would recommend (the vaccine) to a lot of people that don’t want to get it, and a lot of those people voted for me, frankly.”

While the researchers did not have control over where the advertisement would appear on YouTube, the website’s advertising platform Google Ads placed it most often on videos posted by Fox News, including those featuring personalities like Tucker Carlson and Laura Ingraham who have expressed skepticism over the vaccines’ efficacy and safety.

The advertisement also played before videos on YouTube channels run by MSNBC, NBC News and Saturday Night Live. Ultimately, it was placed before some 6 million unique viewers, according to the study.

“We believe that, as long as Americans on the political right are a significant bastion of hesitancy, Donald Trump’s support for vaccination will represent a potent tool that public health messengers can use,” the researchers said in the study.