The agreement with Korea became effective on 4/1/01.

Workers whose employment or self-employment would otherwise be covered by both the U.S. and Korean Social Security systems are covered and taxed by only one country's system beginning 4/1/2001. The agreement doesn't affect a worker's coverage prior to that date. If a worker had earnings before 4/1/2001 that were dually covered, those earnings would be subject to Social Security taxes in both countries.