Scientists in Hong Kong reported last month what many had long suspected could happen. Someone who had recovered from Covid-19 caught the coronavirus again. Since then, about a dozen cases of re-infection have been reported worldwide. These cases demonstrate that a natural infection doesn’t lead to lasting protection, and that the pandemic could persist in the human population. Bloomberg News senior editor Jason Gale talked to health experts about what this means for our ability to stop the virus and to produce an effective immunization.