This is one of the benefits of having the COVID pandemic under control, the South Koreans can watch other nations be guinea pigs before conducting mass vaccination of their own people:

Health authorities said the government decided to up the vaccine supply from 30 million people to 44 million to better fight the virus, but it won’t be in a hurry to inoculate people due to uncertainty over the vaccines’ safety and efficacy.
“As the outbreak in South Korea has remained within a manageable level, I think the government doesn’t have to hastily deploy vaccines like European nations and the United States as efficacy and safety concerns remain,” Lee Hoan-jong, an emeritus professor of Seoul National University, said during the briefing.
“We can secure enough vaccines to deploy them in different groups after closely monitoring potential side effects that may be discovered two or three months after vaccination in other countries.”
Yonhap
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