Wearing a hazmat suit and a face shield, Yang Nan sanitizes his gloved hands each time he passes through one of the four doors that lead to a contaminated area at a local fever clinic.

Purple sterilizing lamps illuminate spaces in between.

"Make sure one door is tightly closed before another opens," said Yang, a respiratory physician who oversees the clinic at the Third Hospital of Hebei Medical University in Shijiazhuang, Hebei province.

Those coming to the clinic typically arrive on their own after developing fevers at home, or are determined to have high temperatures within designated quarantine facilities and are then taken to the clinic for further tests.

In the clinic, masked visitors sit spaced apart in a rectangular zone on the ground floor, waiting to be called to one of the adjacent consulting rooms.

Dozens of wards upstairs are filled with feverish patients. One companion maximum per patient is the facility's rule.

Insulated from other hospital departments, fever clinics have mushroomed nationwide since last year to detect coronavirus patients, who usually exhibit symptoms including high temperatures.

As Shijiazhuang was hit by a recent outbreak, clinics' roles in contagion intervention have come to the fore.

Yang said the clinic works like a filter for those suspected of having COVID-19 during the winter season.

Visitors are required to take nucleic acid tests at least twice before they are cleared to leave.

The test results, coupled with a checkup and an inquiry of contact histories, help doctors determine whether visitors can be safely transferred to regular hospital wards for further treatment.

"There were two visitors who tested positive for coronavirus so far this month," Yang said, adding that the patients have been moved to designated hospitals for COVID-19 in what is called "closed-loop management".

The status of such clinics as outposts for detecting coronavirus patients was cemented by a National Health Commission circular in June.

The document required such clinics test all visitors and place them under isolation before results come back to prevent major outbreaks.

It also warned against turning away such patients, asking clinic employees to shoulder their duties as "gatekeepers" and to "report, treat and transfer" such patients as required.

Individuals and institutions could face legal consequences if they fail to perform required tasks, the circular said.

Dou Jian, vice-president of the hospital overseeing epidemic control work, said the clinic helps screen possible COVID-19 infections at an early stage, which helps prevent in-house infections and exhaustion of medical resources.

"We had 15 isolation wards and added another 12 to cope with increasing patients during the winter. We are still planning to expand capacity," Dou said.

Hebei identified 18 local confirmed cases and three asymptomatic carriers on Thursday, increasing its tally of patients under treatment and asymptomatic carriers under medical observation to 835 and 144, respectively, the Hebei Health Commission reported on Friday morning.

The number of newly confirmed cases for a single day in the province has remained below 50 for four consecutive days.

Dou said the downward trend is reassuring to tens of thousands of front line medical workers, and testifies to the fact that control and isolation measures are working.

"I believe this latest round of cases will be under control very soon," he added.

In one consulting room, physician Wu Xia had just concluded her four-hour shift checking patients and investigating contact histories.

"I have seen about seven patients today and none displayed a significant chance of being infected by the coronavirus," Wu said.

Liu Yueqiao, 29, who patrols the clinic for breach of safety rules, said: "Patients catching a cold or having other suspicious symptoms have to come to us first. We filter them."