“I was a bit dismayed when former president Trump came out and made that statement, and his followers booed him; I was stunned by that,” Fauci told Jonathan Karl, anchor of ABC’s “This Week,” on Sunday.

Trump said he’d received a booster during a live appearance in Dallas with former Fox News host Bill O’Reilly on Dec. 20. O’Reilly said both he and the former president were vaccinated and asked Trump if he was boosted. “Yes,” the president said. The crowd responded with boos, according to video of the exchange that circulated on social media. “That’s all right, it’s a very tiny group over there,” Trump said, in an effort to downplay the audience outburst.

In an interview the next day with Candace Owens on conservative talk show “The Daily Wire,” Trump continued to extoll COVID-19 vaccines.

“The results of the vaccine are very good, and if you do get [COVID-19], it’s a very minor form. People aren’t dying when they take the vaccine,” Trump said.

When Owens pushed him on whether the vaccines were effective, Trump said those who get very sick and are hospitalized are the unvaccinated ones. “But it’s still their choice, and if you take the vaccine, you are protected,” he said.

Trump’s endorsement of three COVID-19 vaccine doses contrasts his earlier vow to not receive a booster shot and his seeming refusal to promote the virtues of the vaccines his administration helped develop. Vaccination rates trend along partisan lines with Republicans far less likely than Democrats to have taken the jabs. A September study by the Kaiser Family Foundation found that 52.8% of people in counties that voted for Biden were fully vaccinated, compared to 39.9% of Trump-won counties. Critics have urged Trump to use his influence in the Republican party to help improve vaccine uptake.

Yet the audience’s negative response to Trump’s booster disclosure may indicate that even the de facto head of the Republican party can’t sway some vaccine holdouts. “Given how popular he is with that group, that they would boo him tells me how recalcitrant they are about being told what they should do,” Fauci said.

Still, Fauci was encouraged by Trump’s promotion of the vaccines and his decision to share his own boosted status. “I hope he keeps it up,” Fauci said.

There’s likely some reputational upside to Trump pushing COVID-19 vaccines. His administration, which was in the White House when the virus first reached U.S. shores, is credited with establishing Operation Warp Speed, a campaign that expedited the timeline for creating some of vaccines that are now the best defense against COVID-19.