Is a One Meter Social Distance Safe Against COVID-19 Infection?

Under the epidemic, should people wear ordinary masks when going out? This topic is becoming an international hot spot. At the epidemic briefing on April 1, WHO Director-General Tan Desai said, "We are still continuing to study evidence of wearing masks." According to the BBC, David Heymann, professor of epidemiology at the London School of Hygiene and Tropical Medicine, said they were concerned about a study from the Massachusetts Institute of Technology that showed people's cough can spread the virus 6 meters, while sneezing can spread up to 8 meters.

Figure. High-speed video imaging of sneeze, green is the trajectory of larger droplets, red is the smaller droplets. Small droplets can be spread farther in the cloud and even enter the room's ventilation system. The recommendation of various authorities is that if you want to sneeze, you need to cover it with paper towels or elbows to prevent the spread of high-speed clouds. (Source: MIT)

David Hyman said the study showed that droplets from coughing and sneezing may spread farther than originally thought. If the evidence is supported, wearing a mask may be as effective as maintaining social distance, or even more effective. David Hyman is also the chairman of the WHO Infectious Disease Strategy and Technical Advisory Group. He said that WHO is evaluating new research from the Massachusetts Institute of Technology and other institutions and will meet in the next few days to discuss whether to modify the recommendation to wear masks.

A sneeze spreads as far as 8 meters

This article was published in the Journal of the American Medical Association (JAMA) on March 26. The author is Lydia Bourouiba, an associate professor from the Massachusetts Institute of Technology ’s Disease Transmission Fluid Dynamics Laboratory. ). According to the article, according to modern standards, the disease transmission model previously developed based on the 1930s seems to be too simplified. The virus can only spread when it stays in the droplet. According to previous opinions, when coughing or sneezing, the large droplets exhaled by people's mouth will quickly settle around, and the small droplets will evaporate faster. Distance can prevent the spread of oral pathogens. Lydia Bruba believes that these views do not consider the high-speed flight of droplets in coughing or sneezing.

As early as 2014, Lydia Brubar studied droplets in sneezes. The high-speed video recorded by her at 8000 frames per second shows that the ejected droplets are ejected from the mouth at high speed in the form of clouds and mist, and the clouds will also lift the droplets. The larger droplet can spread 8 meters, while the droplet can spread 6 meters when coughing. This distance is enough to spread to the other end of the room, and can also enter the ceiling ventilation system. This study used high-speed cameras and other sensors. High-speed video shows that the saliva and mucus mist sprayed from the mouth will appear as water droplets, and all the droplets will form mist, which may be suspended in the air for a long time. The extremely small droplets can travel farther in the clouds.

The current recommendation of the World Health Organization is to maintain a social distance of at least 1 meter from anyone who coughs or sneezes to avoid the risk of infection, while the United Kingdom and the United States recommends a social distance of at least 2 meters. Lydia Bruba ’s point of view is that the previous conclusion of a safe distance of 1 meter or 2 meters was not quantified, but was estimated based on the fact that droplets were dropped to the side by pure gravity attraction. Incorrect. The BBC reported that this research may mean that the safety distance of social alienation may need to be adjusted. However, it is obviously impractical to maintain a social distance of 6 meters or 8 meters.

Lydia Bruba believes that when facing an infected person, wearing a mask helps to keep the airflow and the viruses it contains away from your mouth. Although a thin mask can't filter virus particles, but can allow direct airflow to the side, then in a poorly ventilated indoor environment, wearing a mask can reduce the risk of infection. But Dr. Anthony Fauci, a member of the White House working group and director of the National Institute of Allergy and Infectious Diseases, does not agree with this study. He pointed out at the White House press conference that Lydia Brupa ’s model is unrealistic. He said that for the droplets in the sneeze to spread for 8 meters, it would take a strong person to sneeze or cough very strongly. He said, "I am disturbed by the report because it is misleading."

What other research tips

On April 2nd, a German study published in the journal Nature reported that after analyzing 9 adult patients with neocoronal pneumonia in Germany, it was found that during the initial stage of infection, there was a large amount of viral replication and shedding in the upper respiratory tract. The patient's upper respiratory tract replicates actively, and the virus may “detoxify” at a higher level within the first week after symptoms appear. Two of the patients with early symptoms of pneumonia continued to shed a large amount of virus in their sputum 10 days after the symptoms appeared. After the symptoms disappear, viral nucleic acids can still be detected in the sputum. This also confirms the conclusion that "mild patients are infectious" found by previous scholars.

After the publication of this paper, some media believe that this may imply that a mask should be worn to prevent mild infections. However, there are also views that maintaining social distance in daily life has a positive meaning, and wearing a mask cannot replace maintaining social distance. A study by the University of Hong Kong believes that wearing masks may have a greater risk of pollution. The study found that the new coronavirus can survive 7 days on the outer layer of surgical masks and 2 days on the surface of clothing. According to the Ming Pao report, Pan Liewen, a professor at the School of Public Health of the Hong Kong University School of Medicine who was responsible for the research, said that the public are advised not to reuse masks as much as possible.
Pei Weishi, a professor of virology at the School of Public Health at the University of Hong Kong Medical School, also said that the new coronavirus is more stable in the environment, so it is very important for citizens to wash their hands after touching the mask.

Europe and the United States are ready to modify the mask recommendations

According to a report in the Security Times, it is expected that the Trump administration will soon announce that all Americans should wear masks or other forms of masks in public. A federal official said that everyone would be advised to wear masks in public places such as pharmacies and grocery stores to avoid inadvertent transmission of the virus. The federal official said that the US CDC revised the guidelines as required by the White House and the working group.

Until March 30, the US Centers for Disease Control stated that ordinary people did not need to wear masks, because people wearing masks would touch their faces more frequently, which would increase the risk of infection. But the next day, the US Centers for Disease Control said it was considering recommending ordinary people to wear masks in public places, and Trump also said at the White House press conference that it was considering promoting people to wear masks in public. The trouble is that there is not enough mask supply. According to a survey released on Friday by the US Mayors Conference, about 92% of the 213 cities do not have enough masks for emergency personnel and medical personnel. Anthony Fodge also expressed this concern in an interview with CNN. The recommendation to promote the wearing of masks may lead to a more serious shortage of medical masks such as N95, because medical staff are the people who need masks most.

The mayor of Los Angeles is already urging residents of the city to wear homemade non-medical masks and even handkerchiefs when shopping or doing other essential things. California local health officials have made similar recommendations. Robert Redfield, director of the US Centers for Disease Control, warned that as many as 25% of people with new coronavirus infections may be asymptomatic. This data allows the CDC to review the guidelines for wearing masks. But even in the White House there are different voices. Deborah Birx, coordinator of the White House Fighting Coronavirus Working Group, emphasized that maintaining social distance is still the key, not wearing a mask. Singapore, which has consistently demanded that healthy people do not need to wear masks, has also made changes. According to an editorial in Lianhe Zaobao, "In view of the worsening of the current epidemic and our better understanding of asymptomatic infected persons, we should study the masks for specific people in specific places and locations more thoroughly and thoroughly. Preventive effect. "

The mask must be worn correctly

Regardless of the proposed modification, priority must be given to the use of N95 masks for first-line medical personnel. Prof Ben Cowling, a professor at the School of Public Health at the University of Hong Kong, said in an interview with the Guardian that WHO may encourage the public to use homemade masks or cloth masks while giving priority to ensuring the supply of N95 masks for medical personnel. Even if the revision proposal is made, the mask is only one of the tools for anti-epidemic. Large-scale testing, contact tracing, and isolation of potential infections are all options for anti-epidemic tools. David Hyman warned that the mask must be properly worn and sealed at the nose; if the mask becomes wet, it must be discarded because the virus particles can pass through; people must carefully remove it to avoid contamination of the hands. It is especially important to note that wearing a mask for a long time will cause people to become numb to the above suggestions.

WHO Director-General Tan Desai said that the focus of WHO is to ensure that first-line medical staff can obtain the necessary medical masks and masks with exhalation valves and other personal protective equipment. Tan Desai said that while the debate about the use of masks in the community continues, WHO recommends that sick people and those who care for them wear medical masks. WHO will continue to collect all relevant evidence and continue to evaluate the question of whether masks can be used more widely to control the spread of new coronaviruses in the community, “based on the best scientific evidence and protecting all without fear or favour Human health. " He emphasized, "This is a very new virus, and we are still learning. As the pandemic situation evolves, the evidence is also developing, and our recommendations are also updated.