Malaysia shut down a mass vaccination centre on Tuesday after more than 200 medical staff and volunteers tested positive for the coronavirus.
Science Minister Khairy Jamaluddin said it was difficult to determine if the infections occurred at the centre in central Selangor state, but stressed that swift government action had stopped the cluster.
He urged people who had been vaccinated at the centre from Friday onwards to self-isolate for 10 days in case they develop symptoms.
Mr Khairy, who is in charge of the national immunisation programme, said he ordered testing for all 453 workers at the centre after two volunteers contracted the virus.
He said the 204 people whose results were positive had low viral loads, meaning the amount of virus in their bodies was small.
The centre was shut for deep sanitisation and all its workers were being isolated.
Mr Khairy said the centre will reopen on Wednesday with a new team of medical workers.
Despite a strict lockdown since June 1, the pandemic had worsened in Malaysia, with total confirmed cases exceeding 844,000 with more than 6,200 deaths. But vaccination has picked up pace, with nearly 11% of the population inoculated.