SEOUL, South Korea — South Korea is allowing younger adults who aren’t yet eligible to get coronavirus vaccines to use smartphone apps to sign up for spare doses as officials try to speed up vaccination.

Health officials didn’t immediately say how many people applied for leftover vaccines after the services went live on Thursday afternoon. But a flood of requests temporarily forced mobile chat service Kakao to reboot its servers, said Kim Ki-nam, an official from the Korea Disease Control and Prevention Agency.

Anyone 30 years or older could also use the mobile services of Naver, the country’s biggest website, to register on standby lists.

South Korea has so far administered first doses to just over 4 million people, which is less than 8% of its population. Priority groups have included medical workers, people in long-term care settings, and adults 75 and older.

Officials are hoping that the pace of vaccination will pick up in the coming weeks as they start to inoculate people in their mid-60s and early 70s.

South Korea reported 629 new cases of the virus on Thursday, bringing its caseload to 138,311, including 1,943 deaths.