Ahead of reopening, hundreds of cars formed lines stretching back kilometres from the border at the Mexican city of Tijuana, while queues at pedestrian crossings grew steadily.

Still, differing rules over coronavirus vaccines threaten to hold up family reunions, while the prospect of some curbs easing has also encouraged migrants to try their luck seeking US asylum, posing a new test for the Biden administration.

Maria Luisa Gonzalez, a California resident who visited Tijuana on Friday to take her Chihuahua puppy to the vet and see relatives, was losing patience as she waited to drive back through San Diego at the San Ysidro port of entry on Sunday.

“The operations to speed things up aren’t working,” Gonzalez said, visibly frustrated. “The road diverted me twice, the signs they posted are very confusing.”

Anticipating heavy road use after reopening, Tijuana city council this week said it had re-routed traffic in some streets, but some residents were unclear where to go.

Tijuana’s border with San Diego is one of the busiest in the world, with thousands crossing to work, study or shop daily.

But some inoculated Mexicans will not be able to enter the US immediately if they received vaccines in Mexico that have not been approved by the World Health Organization such as China’s CanSino and Russia’s Sputnik V.

“I never imagined that because I got the CanSino vaccine I wouldn’t be able to cross,” lamented Donato Suarez, a driver at a private university in Tijuana who had hoped to visit relatives in the US he has not seen for nearly two years.

“We even had plans to do something when the border reopened,” he added, noting around 300 people where he works are in the same predicament. “We’ll have to wait.”