But Mr. Trump was notably absent. One reason for the partisan divide over vaccination, experts said, is the president himself. His repeated denigration of scientists and insistence that the pandemic is not a threat, they said, have contributed to a sense among his followers that the vaccine is either not safe or not worth taking.
“I just don’t feel there’s been enough research on it. I think it was sped up too fast,” said Mark Davis, 42, a disabled worker in Michigan. “You don’t even really know the side effects, what’s in it.”
Mr. Lofgren agreed. “The jury’s out on whether it’s going to work,” he said, despite studies showing that the Pfizer and Moderna vaccines were more than 94 percent effective.
Experts say that “herd immunity” — the point at which so many people are immune that the spread of a virus is diminished — can be achieved when roughly 75 percent of the population is vaccinated. While the Trump administration is rolling out a public relations campaign to encourage people to get vaccinated, the reluctance among even a minority of Republicans is deeply troubling to public health experts.
Mr. Trump has been quick to claim credit for the manufacturing and distribution of the vaccine. “Distribution to start immediately,” he said Friday on Twitter, a day after an F.D.A. expert advisory panel recommended approval of Moderna’s vaccine.
Although the president has recovered from Covid-19, he remains vulnerable to reinfection. Dr. Anthony S. Fauci, the government’s top infectious disease scientist, has recommended that Mr. Trump be vaccinated. But he has given no indication that he will actually do so, and he has said little, if anything, to encourage Americans to get vaccinated.
“We need him taking a proactive role,” said Matthew Motta, a political scientist at Oklahoma State University who studies politics and vaccine views, adding, “The single best person to convince you to change your mind about something is somebody who agrees with you, somebody who you trust on other issues.”