Photo:CFP

A medical worker at a hospital's enclosed quarantine ward in Northwest China's Shaanxi Province was confirmed as a COVID-19 case on Thursday, ending the Chinese mainland's 31-day streak of reporting zero new domestically-transmitted cases. 

A preliminary investigation showed that accidental exposure in the enclosed and isolated ward caused the infection. Experts said the chance of causing wider infections is low, shrugging off concerns that the inefficacy of COVID-19 vaccines may have contributed to the infection.

Prior to this single case, the Chinese mainland had reported zero cases for 31 days since February 15.

The medical worker was responsible for collecting nucleic acid test samples of COVID-19 patients and carrying out laboratory tasks at labs in No.8 Hospital in Xi'an, one of the designated hospitals for treating COVID-19 patients in the city, according to the local health authorities on Thursday. 

The hospital has eight imported cases and five asymptomatic cases under quarantine and observation so far.

Before being confirmed on Thursday, the medical worker had been working at the quarantine ward at the hospital since March 4, which is enclosed and has no contact with the outside world, the health authorities said. 

The laboratory worker tested negative for novel coronavirus on March 11 in weekly routine testing but showed symptoms including headache, fever and an infected CT lung image on Wednesday. Two later tests came back positive. 

All the other staff members at the hospital have returned negative results in their nucleic acid tests, and measures including environmental sampling and disinfection have also been carried out for further COVID-19 prevention and control, health authorities said. 

The quarantine ward at the hospital is a designated area for confirmed COVID-19 patients only, where fences have been installed to segregate it from other parts of the hospital. 

Before medical staff members enter the closed-loop zone, they must first test negative and then undergo daily body temperature checks and weekly nucleic acid tests. When they complete a 21-day working shift, they receive a CT scan, four rounds of tests and 14 days of quarantine before they are released, according to a release from Xi'an municipal government. 

Yang Zhanqiu, deputy director of the pathogen biology department at Wuhan University, gave assurances that the risk of the infection spreading is "extremely low." 

"Since the case was accidentally exposed to coronavirus in a closed isolation ward, had been working long hours and closely connected to virus collection and examination, there is little possibility that the virus will spread among the public," Yang told the Global Times on Thursday. 

"Nevertheless, close contacts, including the colleagues who also work in the closed ward, should be strictly monitored," he added.

The case has drawn speculation from Chinese netizens over the efficacy of COVID-19 vaccines, since all medical staff, especially those doing coronavirus-related work, should have been inoculated. 

In response to the concerns, Yang replied that there is no vaccine in the world that can fend off the virus 100 percent effectively. "The high density of the virus in the closed lab and long hours the worker had spent in it both contributed to the contraction."

Yang also called upon the hospital to reexamine the management loopholes in lab seal-off and disinfection to avoid such accidents from happening again.