Chile on Wednesday began administering COVID-19 booster shots to those already immunized with CoronaVac, in a bid to ensure the success of one of the fastest mass vaccination campaigns in the world.

The South American country is offering a dose of AstraZeneca’s vaccine to citizens aged 86 and over who had their first vaccinations before March 31.

Queues of elderly citizens eager for the vaccines began forming at vaccination centers in neighborhoods of the capital, Santiago, on a cold winter morning.

“They arrived very early, like on an election day, very well dressed, very happy,” said Rodolfo Carter, mayor of the La Florida commune on the outskirts of the city. “I think it’s a great sign of hope.”

Chile‘s strong campaign had more than 67 percent of its population fully vaccinated, predominantly with CoronaVac, from Chinese manufacturer Sinovac. But officials said last week that studies showed a booster dose was needed to boost immunity.

“Studies have shown that in approximately six months there is a decrease (of antibodies) and that’s why we decided … to give that booster dose,” Health Minister Enrique Paris told reporters.

Chile joins the United States, Germany, France, and Israel in administering the booster shots, despite the World Health Organization’s call to wait until more people around the world can receive the first dose.

(Translated by LABS)