Risk factors

Risks: how to recognize and avoid them

Erin Bromage, The Risks – Know Them – Avoid Them Translated by Tatiana Lando, XX2 century

For links, see the translation or the original of the article.

The President of the Russian Federation suggested that the regions develop measures for a gradual exit from the self-isolation regime. Not so long ago, quarantine restrictions began to be lifted in various US states. Similar situations give rise to similar questions and problems. Will there be a risk of infection after the restrictions are lifted? How safe is it now to go to the office, to the store, to visit a restaurant, to use public transport, to talk to someone on the street? We present to your attention a translation of an article by an American biologist Erina Bromage, in which he examines these problems in detail, with examples, and answers these questions.

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It seems that many people are breathing a sigh of relief, but I do not know why. The epidemic curve has a relatively predictable rise, and as soon as it reaches a peak, it is possible to predict a descent. We have reliable data on outbreaks in China and Italy, which show how the number of deaths is slowly decreasing, but mortality persists for several months. If the peak was 70,000 deaths, then it is possible that over the next six weeks, while the curve will decrease, we will lose another 70,000 people. And this is subject to quarantine.

As the states open, the virus will get more fuel, which means that the forecasts will lose relevance. I understand why there is a desire to restart the economy, but, as I said earlier, if the biology issue is not resolved, the economy will not recover.

Only a few states show a steady decline in the number of new infections. Indeed, as of May 3 (in the figure below), the number of patients is growing in most cases, but they are removing quarantine. Here is a simple example of a trend in the US: if we exclude the data for New York and look at the rest of the US, it turns out that the number of new cases per day is increasing. Total: the only reason why the graph of the total number of new cases in the United States now seems to be flat is that the epidemic in New York was very large-scale, but now it is under control.

Thus, in most of the United States, we will add fuel to the viral fire by removing restrictions. This will happen independently of me, so the purpose of my post is to help you avoid excessive risks.

Where do people get sick? 

We know that most people get infected at home. Household members become infected with the virus in society and bring it home, where constant contact between household members leads to infection.

But how do people get infected in society? I regularly hear people worrying about grocery stores, bike rides, irresponsible runners without masks… Are these reasons for concern? Actually, no. Let me explain.

In order to get infected, you need to get an infectious dose of the virus. Based on studies of infectious doses with MERS and SARS, some believe that only 1,000 SARS-CoV-2 virus particles are required for infection. Note that this still needs to be determined experimentally, but you can use this number to demonstrate how infection can occur. You can get infected by inhaling 1000 viral particles once, or by rubbing your eyes only once, or by inhaling 10 times 100 viral particles, or by inhaling 10 times 100. Each of these situations can lead to infection.

How much virus gets into the environment?

Latrines: there are quite a lot of surfaces in latrines that people often touch – door handles, faucets, entrance doors. Thus, the risk of transmission of the virus through surfaces in this environment may be high. It is still unknown whether the virus is excreted fecally or only its "fragments", but it is known that flushing the toilet leads to the aerosolization of many drops. Be especially careful in public toilets (both with surfaces and with air) until more data becomes available.

Cough: When a person coughs once, he releases about 3,000 drops, and the drops spread at a speed of 50 miles per hour (80.5 km/h). Most droplets are large and settle quickly (due to gravity), but many remain in the air and can cross the room in a couple of seconds.

Sneezing: When a person sneezes once, he releases about 30,000 drops, while the drops move at speeds up to 200 miles per hour (322 km/h). Most drops are small and spread over long distances (across the room – with ease).

If a person is infected, drops from a single cough or sneeze can contain up to 200,000,000 (two hundred million) viral particles that may end up in their environment.

Breathing: One exhalation releases 50-5000 drops. Most of these drops are slow and settle to the ground quickly. When breathing through the nose, even fewer drops are released. It is important to note that due to the lack of exhalation force during breathing, viral particles are not removed from the lower respiratory tract.

Unlike sneezing and coughing, which emit a huge amount of viral material, droplets released during breathing contain quite a little virus. Specific numbers for SARS-CoV-2 are not yet known, but we can be guided by knowledge about the flu. We know that a person infected with influenza secretes about 3-20 copies of viral RNA per minute of respiration.

Remember the formula: infection = exposure to the virus × time

If a person coughs or sneezes, 200,000,000 viral particles spread everywhere. Some viruses hang in the air, some settle on the surface, most fall to the ground. So if you're talking to a person face to face and that person sneezes or coughs right at you, it's pretty obvious that you can inhale 1,000 viral particles and get infected.

But even if a person coughed or sneezed to the side, some infected droplets – the smallest ones - can hang in the air for several minutes, filling the entire space of a small room with infectious viral particles. That is, it will be enough to enter this room a few minutes after coughing/sneezing something and take a few breaths in order to get a dose of the virus sufficient for infection with a high probability.

With normal breathing with the release of 20 copies per minute into the environment, and when each particle of the virus enters the lungs, 1000 copies will be required, divided into 20 copies per minute = 50 minutes.

Talking increases the release in drops by about 10 times: ~200 copies of the virus per minute. Assuming that all the virus particles are inhaled, it will take ~5 minutes of face-to-face conversation to get the necessary dose.

The formula "virus exposure × time" is the basis of contact tracking. Anyone you spend more than 10 minutes face-to-face with is potentially infected. Anyone with whom you have spent a long time in the same room (say, in the office).

That is why it is especially important that people with symptoms stay at home. Sneezing and coughing emit so many viruses that they can easily infect an entire room of people.

What is the role of asymptomatic carriers in the spread of the virus?

People with symptoms are not the only way to spread the virus. It is already known that at least 44% of all infections – and most transmitted within society – originate from people without any symptoms (asymptomatic or pre-symptomatic people). People can release the virus into the environment 5 days before the onset of symptoms.

Infectious people are found in all age groups, and they all secrete different amounts of the virus. The figure below shows that regardless of age (X-axis), a person can secrete few or many viral particles (Y-axis).

The amount of virus released by an infected person varies during the course of the disease and differs from person to person. The viral load usually accumulates until the moment when a person shows symptoms. Thus, shortly before the onset of symptoms, the patient releases most of the virus into the environment. Interestingly, the data show that only 20% of infected people are responsible for 99% of the viral load that can potentially enter the environment.

So, it's time to get to the point. What are the personal risks of loosening quarantine?

Which major disease outbreaks come to mind first? Most will remember cruise liners. But this is not true. To date, the ships do not even get into the top 50.

If we do not consider the horrific outbreaks in nursing homes, it turns out that most infections occur in prisons, religious buildings and workplaces such as meat processing plants and call centers. Any environment with poor air circulation and high density of people can create problems.

Here are some examples of the super-spread of the virus:

• Meat industry: Meat processing plants have a high density of employees, and they must interact with each other in the deafening noise of industrial equipment and in cold rooms that preserve the virus well. Currently, outbreaks have been recorded at 115 factories in 23 states, more than 5,000 workers have been infected and 20 have died.
• Weddings, funerals, birthdays: 10% of infections at the beginning of the epidemic.
• Conferences: Personal business communication, for example, the Biogen conference in Boston in March. Let's see what can happen when we get back to work or go to a restaurant.

Restaurants: Several examples of field epidemiology have clearly demonstrated how one asymptomatic carrier in a restaurant affects others (see below). The infected person (A1) had dinner at the same table with nine friends. Dinner lasted from one to one and a half hours. During dinner, an asymptomatic carrier, just breathing, threw a low level of the virus into the air. The air flow (from the vents in the restaurant) moved from right to left. About 50% of the people at the infected person's table got sick over the next seven days. 75% of the people from the next table, who were sitting on the movement of the air, became infected. And even two out of seven people from the table on the other side of the airflow were infected (presumably due to the turbulent airflow). No one at tables E or F was infected, they were outside the main airflow coming from the air conditioner on the right to the exhaust fan on the left.

Offices: another eloquent example is the flash in the call center (see below). One infected worker came to work on the 11th floor of the building. There were 216 employees on this floor. During the week, 94 of them became infected (43.5%: blue chairs). 92 of these 94 people became ill (only two did not show symptoms). Please note that one side of the office was infected in the first place, and few people were infected on the other side. It is not known exactly how people were infected: by respiratory droplets / inhaling viral particles or by transmission through surfaces (door handles, water coolers, elevator buttons, etc.). But I would like to emphasize that being in a confined space with the same air for a long period increases the likelihood of infection. Three more people on other floors of the building were infected, but the authors were unable to link the infection to the primary cluster on the 11th floor. Separately, it is interesting that, despite significant interaction between workers from different floors in the elevators and in the lobby, the outbreak was mostly limited to one floor. This highlights the importance of exposure and timing in the spread of SARS-CoV-2.

Choir: A church choir in Washington State. People knew about the virus and took steps to minimize transmission: for example, they avoided the usual handshakes and hugs, brought their notes so as not to use them together, and socially distanced themselves during the service. One asymptomatic carrier infected most of those present. The choir sang for two and a half hours in a closed church about the size of a volleyball court.

Singing to a greater extent than talking, sprays respiratory drops. Deep breathing during singing facilitates the penetration of respiratory droplets into the lungs. Two and a half hours turned out to be enough for people to be exposed to the virus in the quantities necessary for the infection to occur. Within four days, 45 of the 60 members of the choir developed symptoms of the disease, two died. The youngest infected was 31, the average age was 67 years.

Indoor sports: perhaps this is a uniquely Canadian case – a super-spread occurred during a curling competition in Canada. 72 people took part in the event, and it became a "hot spot" for transmission of infection. During the curling game, the participants of the competing teams are in close contact with each other in a cool closed environment for a long time and can breathe heavily. As a result of this tournament, 24 out of 72 people were infected.

Birthdays / Funerals: to illustrate how simple infectious chains can be, I will tell you a real story that happened in Chicago; the name has been replaced with a fictional one. Bob was infected, but didn't know about it. Bob shared a takeaway meal served in a shared dish with two family members. Dinner lasted three hours. The next day, Bob attended the funeral, where, expressing condolences, he hugged family members and others present. Within four days, both family members with whom he had dinner fall ill. The third family member who hugged Bob at the funeral also got sick. But Bob didn't stop. He attended the birthday party with nine other guests. They hugged and ate together at the party for three hours. Seven of them got sick. A few days later, Bob fell ill, was hospitalized, got on a ventilator and died.

But Bob's legacy lived on. Three of the people infected with the Bean at the birthday party went to church, where they sang, passed a begging bowl, etc. Members of this church got sick. In general, Bob directly infected 16 people aged 5 to 86 years. Three of those 16 died.

Transmission of the virus within the household and back into society – at funerals, birthdays and church meetings – is believed to be responsible for the wider spread of COVID-19 in Chicago.

Sobering, isn't it?

What do flashes have in common

I have highlighted these outbreaks to show what the different COVID-19 outbreaks have in common. All these cases of infection occurred in rooms where people were close to each other, talking a lot, singing or shouting. The main sources of infection are the home, office, public transport, events and restaurants. This is about 90% of all infections. Stores, on the contrary, are responsible for a small percentage of infections that have been traced.

It is important to note that in countries where contacts are properly monitored, only one outbreak in the open air was recorded (less than 0.3% of the detected infections).

So, back to the original thought of my post

Rooms for a large number of people with limited air exchange or air recirculation are of the greatest concern in terms of transmission of the virus. We know that the simultaneous presence of 60 people (choir) in a hall the size of a volleyball court caused a massive outbreak of infection. The same thing happened in the restaurant and call center. Recommendations for social distancing do not help in rooms where people spend a lot of time: people on the opposite side of the room are infected.

The principle is exposure to the virus over a long period of time. In all these cases, people were exposed to the virus in the air for a long time (several hours). Even if they were at a distance of 50 feet (choir or call center), even if the number of viral particles in the air was small, the virus that got in for a long time was enough to cause infection, and in some cases death.

Social distancing is sufficient for protection during brief exposure or exposure on the street. In these situations, there is not enough time to achieve an infectious viral load if you are standing 6 feet apart or where wind and open air repeatedly dilute the concentration of particles and reduce the viral load. Sunlight, heat and humidity affect the survival of viruses – all this reduces the risk of outdoor exposure.

When assessing the risk of infection (through breathing) in a grocery store or shopping center, it is necessary to take into account the volume of air space (very large), the number of people (limited), the time that people spend in the store (employees – all day, customers – an hour). Taken together, for the buyer: low density, a large volume of air in the store and the limited time he spends there means that the probability of getting an infectious dose of the virus is low. But store employees spend longer time there, which makes it more likely to get an infectious dose, and, consequently, work becomes more risky for them.

In fact, as the restrictions loosen, we will begin to go out more often, perhaps even resume work in the office, and you will need to monitor the environment. How many people are here, how much air and how much time I'm going to spend here. If you work in an open-plan office, you need to critically assess the risks (the volume of the room, the number of people and air flows). If you need to talk at work, or, even worse, shout, you need to assess the risk.

If you are sitting in a well-ventilated room where there are few people, the risk is low.

If I'm on the street and walking past someone, remember that "dose and time" are needed for infection. You need to be in someone else's air stream for at least five minutes for the risk of infection. Although runners may release more virus due to the fact that they breathe deeper, remember that exposure time is reduced due to their speed.

Although the post is dedicated to respiratory effects, please do not forget about surfaces. Infected droplets land somewhere. Wash your hands more often and don't touch your face!

As we are allowed to move more freely and have regular contact with more people in more places, the risks for us and family members increase. Even if you can't wait to get back to normal life, do your part and put on a mask to reduce the release of the virus into the environment. This will help everyone, including your business.

This post was inspired by an article written by Jonathan Kay at Quillete: COVID-19 Superspreader Events in 28 Countries: Critical Patterns and Lessons.

About the author

Erin S. Bromage, PhD, Associate Professor of Biology at The University of Massachusetts at Dartmouth (University of Massachusetts Dartmouth). Dr. Bromage graduated from the School of Veterinary and Biomedical Sciences of the University. James Cook, Australia, where his research focused on epidemiology and immunity to infectious diseases in animals. His postdoctoral training took place at the College of William and Mary, the Virginia Institute of Marine Science, in the Laboratory of Comparative Immunology of the late Dr. Stephen Kaattari.
Dr. Bromage's research focuses on the development of the immune system, the immunological mechanisms responsible for protecting against infectious diseases, as well as on the development and use of vaccines to combat infectious diseases in animals. He also develops diagnostic tools for detecting biological and chemical threats in the environment in real time.
Dr. Bromage joined the faculty of the University of Massachusetts at Dartmouth in 2007, he teaches courses in immunology and infectious diseases, including this semester a course on the ecology of infectious diseases, dedicated to the new outbreak of SARS-CoV-2 in China.

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