Most people who are exposed to SARS-CoV-2, the coronavirus that causes COVID-19, experience only mild symptoms or no symptoms at all.
However, the infection can still be transmitted from them to other people, and the overall case fatality rate appears to be approaching 0.5–1.0%.
Therefore, it is important to establish whether people who have once contracted the virus can become infected again and become infectious, or whether they are immune to future infection.