‘We believe in science.’ Washington, Oregon and Nevada join California’s vaccine-review plan.

“Frankly, I’m not going to trust the federal government’s opinion,” Mr. Cuomo said at the time.

But in a news briefing on Tuesday, Mr. Newsom emphasized that the extra review was meant to reinforce federal findings and to ensure that the Western states have planned in detail who should be able to receive what are expected to be very limited early doses. It would not, he said, stall or add an additional layer of politics to the process.

“It will not cause delays,” Mr. Newsom said. “It’s going to increase transparency and trust.”

About two-thirds of Californians surveyed in a recent poll by the Public Policy Institute of California said they were concerned about the development of a vaccine moving too quickly.

California’s health secretary, Dr. Mark Ghaly, added that the review would not involve continuing or duplicating any vaccine trials. Rather, the panel will look at data and other information, “a lot of which is publicly reported, through the eyes of experts,” he said.

The review will allow the states to plan for a complex distribution process in detail and with equity in mind.

“The independent review conducted by this panel of doctors, scientists, and health experts will ensure that a safe and effective Covid-19 vaccine is available to everyone, especially communities that have been disproportionately impacted by this disease,” Oregon’s governor, Kate Brown, said in a statement.