South Korea's rollout of Covid-19 vaccinations to people aged 55-59 has stuttered to a week-long halt after a spike in new cases sparked a rush for shots, booking up available supplies and crashing the official government reservation website.

The halt in vaccination appointments for people in the upper 50s age bracket came late on Monday, the first day that inoculation bookings were opened to under-60s. It came as daily infections, featuring the highly contagious Delta variant, came in at 1,150, a seventh straight day of more than 1,000 cases — the country's worst coronavirus outbreak so far, Reuters reports.

“Due to strong demand, 1.85 million Moderna doses were fully booked and reservations for those who couldn't sign up will resume on July 19,” the Korea Disease Control and Prevention Agency said in a statement.

A total of around 3.52 million people in the 55-59 age group were eligible to sign up, leaving 1.67 million still to reserve shots.