(Photo: CNA)

A few other countries, such as Israel and Germany, plan to or have already administered the third shot to avoid another crisis due to the contagious Delta variant of the coronavirus.

Scientists are still divided over the broad use of COVID-19 vaccine boosters among those without underlying problems as benefits of the boosters remain undetermined.

Pfizer has said the efficacy of the vaccine it developed with BioNTech drops over time, citing a study that showed 84% effectiveness from a peak of 96% four months after a second dose. 

Moderna has also said it sees the eventual need for booster doses, especially since the Delta variant has caused "breakthrough" infections in fully vaccinated people.