that, and the race for a vaccine tonight. >> reporter: tonight, an encouraging turn in the battle against the pandemic. after fears that virus patients in south korea had gotten it a second time, the south korean cdc thinks it's likely that people cannot be infected twice. but that conclusion is still preliminary. and they don't know how long that immunity lasts. those scientists pointing to those relapsing cases, now saying they were actually due to testing failures. >> these results are very encouraging and should be very reassuring to the public. that once you get infected with covid-19, it's unlikely you're going to get infected again. >> reporter: 277 recovered south korean patients had tested positive for a second time along with patients in china and japan. this prompted fears the virus was mutating so quickly that people were not immune to catching it again. the south korean cdc calling those test results false positives. in addition, they're saying