South Korea has begun inoculating healthcare workers and airline crew members on Monday, in an effort to speed up its vaccination drive. 

COVID-19 shots for police, fire officials and soldiers will also be pushed up to start as early as this month.

The changes come as the volume of available vaccines has opened up after AstraZeneca vaccinations of people under the age of 30 were put off over blood clot concerns. However, the global vaccine supply still remains a variable, especially following news that the U.S. is considering "booster shots," or a third vaccine dose.

In consideration of the supply situation, South Korea has increased the interval between the first and second doses of the AstraZeneca vaccine to 12 weeks. Over one-point-five million people have received their first shots so far.

In a virus response meeting Sunday, acting Prime Minister Hong Nam-ki said the goal is to vaccinate three million people by April and 12 million in the first half of the year to achieve herd immunity by November.