The Tokyo Metropolitan Government said Monday it is considering implementing an online lottery system at a walk-in COVID-19 vaccination site for young people in the capital. The announcement came after thousands swarmed the site in its first days in operation.

Concerns are rising that the crowds forming around the vaccination site near JR Shibuya Station — where people line up to get tickets for a limited number of shots offered each day — could create a high-risk environment for spreading the virus.

Details on an online lottery system will be announced after they are finalized, with the current system of distributing lottery tickets from 9 a.m. to 10:30 a.m. to remain for the time being, the metropolitan government said.

The site, set up in the wake of the coronavirus resurgence in the capital, aims to vaccinate people between the ages of 16 and 39 who reside in or commute to Tokyo — a consequence of the faster pace of spread within this age group.

The metropolitan government shifted to a lottery system after difficulties on the first day of the program Friday, when a line began to form from the early morning, with people snapping up the first 200 vaccination slots provided on a first-come, first-served basis — well before registration was scheduled to start.

The vaccination site had intended to distribute lottery tickets from 9 a.m. on Saturday but started about 40 minutes ahead of schedule due to the growing line, which eventually stretched around 1 kilometer, according to Tokyo officials.