The booster dose administration started on Wednesday in Western and Southern Provinces and the Anuradhapura and Ampara districts.
Doctors on the ground told Xinhua news agency that the Pfizer jab is used as the booster dose, and that all people in the cited age group can visit their nearby inoculation centres with their vaccine cards to take the third shot, regardless of whatever vaccine they received as the first or second dose.
The first and second doses administered in Sri Lanka were the Sputnik V, Moderna, Pfizer, AstraZeneca and Sinopharm.
Health officials said people over 60 are eligible for the booster dose if it has been three months since they received the second dose.
Officials also called on people not to queue up at vaccination centres but to await a digital message to be sent to them stating the vaccination time.
According to health authorities, the booster dose rollout in the country started earlier with frontline health workers including security forces personnel, with over 120,000 people having so far received it.
According to official statistics, 93.5 per cent of the population over 20 years of age in Sri Lanka have been fully vaccinated.
As of Thursday, Sri Lanka's overall Covid caseload and death toll stood at 553,722 and 14,057.