The South American country has seen daily infections rise to over 4,000, doubling over the last week, government data show, a reflection of soaring infections globally, despite hopes over data suggesting omicron may be less fatal, if more contagious.
"This vaccine, this fourth dose or second booster dose, will be available to everyone. We start today with immunocompromised people and we will continue with maximum speed," said Chile's President Sebastian Pinera at a hospital in the capital Santiago.
The campaign began with people aged 12 or older with compromised immunity who received their first booster dose up until last September. The process will be extended to the general population of people aged over 55 years in February.
"This shows a great advance in terms of health issues," said Maria Isabel Sandoval, a 61-year-old retired teacher with cancer, who received her dose in a health centre in the municipality of La Florida, in the southeast of the capital.
"It is like being a little bit ahead of things."
Carla Riquelme, 37, a nurse technician who had breast cancer, agreed.
"As infections rise, it is ideal to get vaccinated again, to have more booster doses. I feel calmer," she said.
Since the end of 2020, Chile has fully inoculated more than 14 million people - out of a population of 19 million - while 11.3 million people have already received a third booster dose, according to data from the Ministry of Health.