At the San Ysidro Port of Entry, every available booth was staffed with Customs and Border Protection agents, who checked some people for proof of vaccination before waving most of them through. Only a few booths had been open during the previous 18 months, when a pandemic travel ban kept out most travelers other than American citizens and permanent residents or people with “essential needs.”

Yadira Perdomo, who is Colombian, had received experimental medical treatment in Los Angeles but had not been able to see her doctor there for a follow-up. She crossed the border early on Monday in a wheelchair pushed by her sister Hannah Perdomo.

Some noncitizens were able to receive medical exemptions to enter the United States during the travel ban, but the sisters wanted to cross together. They moved to Baja California two months ago to await the day when the border would open to fully vaccinated visitors. They got in line at the crossing at 3 a.m. Monday.

“I feel very happy to be able to move forward with my life,” Yadira Perdomo said.

In the days before the reopening, there was some confusion among Mexicans over which vaccines would be accepted and what proof would be required.

Maria, who was on her way to see her granddaughter in Los Angeles and declined to give her last name, said she had received the Sinovac vaccine from China. Though the United States hasn’t authorized its use, the World Health Organization has, so it is being accepted at the border.

“I’m going right now because I don’t need permission to, because I can,” she said. “It’s been very sad to be apart.”

People hoping to visit the United States waited for hours last week to apply for vaccination certificates at Health Ministry offices in Tijuana. Mexican officials encouraged people to get the certificates and be included in a national database, even though the vaccination slips given out by doctors when shots are administered would be equally valid for crossing the border.

Carlos Gutiérrez, a dentist, didn’t want to take any chances. He waited in line for a certificate, just in case it would make a difference. “I have a lot of shopping to do — video games, clothes, things you can’t get in Mexico,” he said.

Though all the car lanes at San Ysidro were in use on Monday, only one pedestrian entrance was open. Another, closed throughout the travel ban, is blocked by an encampment of asylum seekers, who still cannot cross freely.

At a news conference, local officials and business leaders in San Ysidro, a section of the city of San Diego, said the reopening was badly needed on both sides of the border.

Jason Wells, executive director of the San Ysidro Chamber of Commerce, said that local businesses had suffered $1.3 billion in lost sales because of border restrictions that he called “discrimination against tourism.”

“We are all truly essential,” he said.