A man not wearing a face mask may have contributed to a community outbreak of COVID-19 involving eight people in the northern Chinese port city of Tianjin, local health authorities said Tuesday.

The man, surnamed Wang, was the first in the local cluster to test positive for the coronavirus on Nov. 10 during a routine screening, according to the authorities. The day before, the cold-chain worker had coughed and sneezed while not wearing a face mask in at least two different elevators in his residential community, potentially spreading the virus to others who later used them with their faces exposed.

The results of the local investigation add to overwhelming evidence suggesting that face masks can reduce the risk of novel coronavirus transmission.

“Scientific research, clinical practice, and years of experience all tell us that masks are effective (in providing protection),” Shen Yinzhong, an infectious disease expert at the Shanghai Public Health Clinical Center, told Sixth Tone.

While China’s health authorities mandated that people must wear masks in public at the beginning of the COVID-19 outbreak earlier this year, public health messaging in parts of the West has been inconsistent. During the early stages of the pandemic, authorities such as the World Health Organization and the U.S. Centers for Disease Control and Prevention did not recommend wearing masks due to a lack of scientific evidence at the time that they could prevent COVID-19 transmission.

As the virus has spread globally, however, a growing body of evidence now suggests that masks do reduce transmission. Multiple health authorities from across the world including the WHO have reversed their previous positions and now recommend universal mask-wearing in public.

In particular, scientists have shown that face masks are effective in blocking the virus from entering the air in respiratory droplets when an infected person talks, coughs, or sneezes. They may also protect healthy people from being exposed to the virus to some degree.

Blocking the source of infection has become an important factor in controlling COVID-19. Research has shown that people who become infected without developing symptoms could account for 17% of all patients, while some might only develop symptoms days after exposure. These two groups — the presymptomatic and asymptomatic — may contribute to half of all transmissions.

“In reality, you don’t know who’s infected and who’s not,” Shen said. “So if all of us wear one (a face mask), we can protect ourselves and others.” He added that face masks are particularly important in confined environments with poor ventilation, such as elevators or cargo containers, which were recently linked to a small outbreak in Shanghai.

The city with over 24 million people reported seven domestic cases over the past week, two of which were traced to a freight container from North America at Shanghai Pudong International Airport. The local health authority said Monday that the two patients had cleaned the container without wearing masks.

Winter and spring are the usual peak seasons for respiratory viruses, such as the coronavirus and influenza, because dry and cold air allows pathogens to spread more easily. While China’s coronavirus situation has been effectively brought under control, Shen said, the recent positive tests of imported cold-chain products and their handlers should serve as a warning.

“There is a continuous risk of imported infections — not only human-to-human, but also object-to-human,” he said. “Keep a face mask on. Don’t take chances by thinking you won’t catch the virus.”

Editor: Bibek Bhandari.

(Header image: Residents line up for nucleic acid tests in Tianjin, Nov. 23, 2020. Wang Yinghao/People Visual)