A Shanghai hospital conducts COVID-19 testing on its staffers on January 21, 2021. (Photo: Global Times)

Two of the six COVID-19 patients that Shanghai found on Thursday, who work at local hospitals, hadn't received COVID-19 vaccination before being infected with the virus, Shanghai's health authority said on Friday, responding to local citizens' concerns about the efficacy of Chinese COVID-19 vaccines.

Shanghai has so far vaccinated 180,000 medical workers, Shanghai Health Commission said on Friday.

The two newly found COVID-19 patients hadn't received a dose, it said.

Medical personnel are among the priority groups for voluntary COVID-19 vaccination along with those working at ports or cold-chain companies, the commission said.

The government doesn't force people to be vaccinated, but "encourages the priority groups to get vaccination to reduce infection risks," Wu Jinglei, head of the commission, said at a local press conference on Friday afternoon.

More than 840,000 people in Shanghai had received COVID-19 vaccination on a voluntary basis as of Friday, according to Wu.

Shanghai is tackling the epidemic resurgence, two months since the last local case was reported in the city. All the six patients found in this round of the virus outbreak are in a stable condition, the commission said.

As of 12 pm Friday, it has traced 112 close contacts of the six patients, and 15,918 people who might have come into contact or had overlapping travel histories with them, it said.

All the 112 close contacts and 15,907 of the 15,918 related people had tested negative for COVID-19 in their first round of nucleic acid testing, and the remaining number are still being tested, said Wu.

A hotel in Shanghai's downtown Huangpu district has been classified as a medium-risk area for COVID-19, the commission said. Among the six local patients, two work at the hotel and one temporarily lived there, it explained.