Chile has started administering fourth doses of Covid-19 vaccines, the first Latin American country to do so, as the Omicron variant fuels a surge in cases.
People with weakened immune systems will be the first to receive the shots. The campaign will be extended to all those over the age of 55 in February.
Infections are rising across South America, which was badly hit by previous waves of the pandemic.
Since then, some countries have managed to achieve high levels of vaccination.
Chile has fully vaccinated more than 14 million people over the age of three, out of a population of 19 million, according to the health ministry. Around 11.3 million people have already received a third dose.
The country's campaign started with the Chinese-made Sinovac vaccine, and the expansion of the programme to include fourth doses will use a combination of Sinovac, Pfizer and AstraZeneca jabs.
President Sebastián Piñera, who visited a Santiago hospital where fourth shots were offered on Monday, also urged people to get vaccinated, saying the jabs are "safe and work".
He said of those refusing to be vaccinated: "What you're doing is irresponsible. If you aren't vaccinated, you'll get infected, you'll infect your families, partners [and] work colleagues."