On May 19, 2020, the South Korean CDC released a study of 285 people who contracted Covid-19, recovered, and then tested positive again. Based on laboratory tests and contact tracing, it found no evidence that any of them were infectious. Viral culture testing was performed for 108 of these individuals, and "all had negative results." Also, "no case was found" in which someone "was newly infected solely from contact with" one of these people. This indicates that their positive Covid-19 test results are merely from dead or non-infectious virus particles. Furthermore, all of these individuals tested positive for "neutralizing antibody production." Thus, South Korea is changing its policy to treat these people as non-infectious.