The Risks – Know Them – Avoid Them
It seems many people are breathing some relief, and I’m not sure why. An
epidemic curve has a relatively predictable upslope and once the peak is reached,
the back slope is also predictable. Assuming we have just crested in deaths at
70k, that would mean that if we stay locked down, we lose another 70,000 people
over the next 6 weeks as we come off that peak. That’s what’s going to happen
with a lockdown.
As states reopen, and we give the virus more fuel, all bets are off. I understand
the reasons for reopening the economy, but I’ve said before, if you don’t solve the
biology, the economy won’t recover.
There are very few states that have demonstrated a sustained decline in numbers
of new infections. Indeed, the majority are still increasing and reopening. As a
simple example of the USA trend, when you take out the data from New York and
just look at the rest of the USA, daily case numbers are increasing. Bottom line:
the only reason the total USA new case numbers look flat right now is because
the New York City epidemic was so large and now it is being contained.
So throughout most of the country we are going to add fuel to the viral fire by
reopening. It’s going to happen if I like it or not, so my goal here is to try to guide
you away from situations of high risk.
Where are people getting sick?
We know most people get infected in their own home. A household member
contracts the virus in the community and brings it into the house where
sustained contact between household members leads to infection.
But where are people contracting the infection in the community? I regularly
hear people worrying about grocery stores, bike rides, inconsiderate runners
who are not wearing masks…. are these places of concern? Well, not really. Let
me explain.
In order to get infected you need to get exposed to an infectious dose of the virus;
the estimate is that you need about ~1000 SARS-CoV2 viral particles for an
infection to take hold, but this still needs to be determined experimentally. That
could be 1000 viral particles you receive in one breath or from one eye-rub, or
100 viral particles inhaled with each breath over 10 breaths, or 10 viral particles
with 100 breaths. Each of these situations can lead to an infection.
How much Virus is released into the environment?
A Toilet flush: Without a seat to close, a single flush releases ~8000 droplets
into the air. If the person using the restroom before you was infected, you have a
chance of contracting the virus via breathing the air in the bathroom. While the
paper in question did not look for live virus, it is clear that infected people are
releasing, at a minimum, viral RNA, in bowel movements. Until further
experiments are done to determine whether is is just viral fragments, or
infectious material, I would avoid public bathrooms or wait a few minutes before
entering so gravity can bring the droplets to the floor.
A Cough: A single cough releases about 3,000 droplets and droplets travels at 50
miles per hour. Most droplets are large, and fall quickly (gravity), but many do
stay in the air and can travel across a room in a few seconds.
A Sneeze: A single sneeze releases about 30,000 droplets, with droplets traveling
at up to 200 miles per hour. Most droplets are small and travel great distances
(easily across a room).
If a person is infected, the droplets in a single cough or sneeze may contain as
many as 200,000,000 (two hundred million) virus particles which can all be
dispersed into the environment around them.
A breath: A single breath releases 50 – 5000 droplets. Most of these droplets are
low velocity and fall to the ground quickly. There are even fewer droplets
released through nose-breathing. Importantly, due to the lack of exhalation force
with a breath, viral particles from the lower respiratory areas are not expelled.
Unlike sneezing and coughing which release huge amounts of viral material, the
respiratory droplets released from breathing only contain low levels of virus. We
don’t have a number for SARS-CoV2 yet, but we can use influenza as a guide. We
know that a person infected with influenza releases about 3 – 20 virus RNA copies
per minute of breathing.
Remember the formulae: Successful Infection = Exposure to Virus x Time
If a person coughs or sneezes, those 200,000,000 viral particles go everywhere.
Some virus hangs in the air, some falls into surfaces, most falls to the ground. So
if you are face-to-face with a person, having a conversation, and that person
sneezes or coughs straight at you, it’s pretty easy to see how it is possible to
inhale 1,000 virus particles and become infected.
But even if that cough or sneeze was not directed at you, some infected droplets–
the smallest of small–can hang in the air for a few minutes, filling every corner of
a modest sized room with infectious viral particles. All you have to do is enter
that room within a few minutes of the cough/sneeze and take a few breaths and
you have potentially received enough virus to establish an infection.
But with general breathing, 20 copies per minute into the environment, even if
every virus ended up in your lungs, you would need 1000 copies divided by 20
copies per minute = 50 minutes.
Speaking increases the release of respiratory droplets about 10 fold; ~200 copies
of virus per minute. Again, assuming every virus is inhaled, it would take ~5
minutes of speaking face-to-face to receive the required dose.
The exposure to virus x time formulae is the basis of contact tracing. Anyone you
spend greater than 10 minutes with in a face-to-face situation is potentially
infected. Anyone who shares a space with you (say an office) for an extended
period is potentially infected. This is also why it is critical for people who are
symptomatic to stay home. Your sneezes and your coughs expel so much virus
that you can infect a whole room of people.
What is the role of asymptomatic people in spreading
the virus?
Symptomatic people are not the only way the virus is shed. We know that at least
44% of all infections–and the majority of community-acquired transmissions–
occur from people without any symptoms (asymptomatic or pre-symptomatic
people). You can be shedding the virus into the environment for up to 5 days
before symptoms begin.
Infectious people come in all ages, and they all shed different amounts of virus.
The figure below shows that no matter your age (x-axis), you can have a little bit
of virus or a lot of virus (y-axis). (ref)
The amount of virus released from an infected person changes over the course of
infection and it is also different from person-to-person. Viral load generally
builds up to the point where the person becomes symptomatic. So just prior to
symptoms showing, you are releasing the most virus into the environment.
Interestingly, the data shows that just 20% of infected people are responsible for
releasing 99% of all the virus into the environment. (ref)
So now let’s get to the crux of it. Where are the personal dangers from
When you think of outbreak clusters, what are the big ones that come to mind?
Most people would go to the cruise ships. But you would be wrong. Ship
outbreaks don’t even land in the top 50 outbreaks to date.
The biggest outbreaks are in prisons, religious ceremonies, and workplaces, such
a meat packing facilities and call centers. Any environment that is enclosed, with
poor air circulation and high density of people, spells trouble.
The biggest super-spreading events are:
Meat packing: In meat processing plants, densely packed workers must
communicate to one another amidst the deafening drum of industrial machinery
and a cold-room virus-preserving environment. There are now outbreaks in 115
facilities across 23 states, 5000+ workers infected, with 20 dead. (ref)
Weddings, funerals, birthdays: 10% of early spreading events
Business networking: Face-to-face business networking like the Biogen
Conference in Boston in March. Or the businessman from Maine who spread the
disease to Malaysia while on a business trip.
As we move back to work, or go to a restaurant, let’s look at what can happen in
those environments.
Restaurants: Some really great shoe-leather epidemiology demonstrated clearly
the effect of a single asymptomatic carrier in a restaurant environment (see
below). The infected person (A1) sat at a table and had dinner with 9 friends.
Dinner took about 1 to 1.5 hours. During this meal, the asymptomatic carrier
released low-levels of virus into the air from their breathing. Airflow (from the
restaurant’s various airflow vents) was from right to left. Approximately 50% of
the people at the infected person’s table became sick over the next 7 days. 75%
of the people on the adjacent downwind table became infected. And even 2 of the
7 people on the upwind table were infected (believed to happen by turbulent
airflow). No one at tables E or F became infected, they were out of the main
airflow from the air conditioner on the right to the exhaust fan on the left of the
room. (Ref)
Workplaces: Another great example is the outbreak in a call center (see below).
A single infected employee came to work on the 11th floor of a building. That
floor had 216 employees. Over the period of a week, 94 of those people become
infected (43.5%: the blue chairs). 92 of those 94 people became sick (only 2
remained asymptomatic). Notice how one side of the office is primarily infected,
while there are very few people infected on the other side. Being in an enclosed
space, sharing the same air for a prolonged period increases your chances of
exposure and infection. The estimates were that 94% of infections were from
respiratory droplets / respiratory exposure, and roughly 6% from fomite
transfer (door handles, shared water coolers, elevator buttons etc). Another 3
people on other floors of the building were infected, most likely from fomite
transfer (doors handles, elevator buttons etc) or from being in an enclosed
elevator with the infected person. (ref)
Choir: The church choir in Washington State. Even though people were aware of
the virus and took steps to minimize transfer; e.g. they avoided the usual
handshakes and hugs hello, people also brought their own music to avoid
sharing, and socially distanced themselves during practice. A single
asymptomatic carrier infected most of the people in attendance. The choir sang
for 2 1/2 hours, inside an enclosed church which was roughly the size of a
volleyball court.
Singing, to a greater degree than talking, aerosolizes respiratory droplets
extraordinarily well. Deep-breathing while singing facilitated those respiratory
droplets getting deep into the lungs. Two and half hours of exposure ensured
that people were exposed to enough virus over a long enough period of time for
infection to take place. Over a period of 4 days, 45 of the 60 choir members
developed symptoms, 2 died. The youngest infected was 31, but they averaged
67 years old. (ref)
Indoor sports: While this may be uniquely Canadian, a super spreading event
occurred during a curling event in Canada. A curling event with 72 attendees
became another hotspot for transmission. Curling brings contestants and
teammates in close contact in a cool indoor environment, with heavy breathing
for an extended period. This tournament resulted in 24 of the 72 people
becoming infected. (ref)
Birthday parties / funerals: Just to see how simple infection-chains can be, this
is a real story from Chicago. The name is fake. Bob was infected but didn’t know.
Bob shared a takeout meal, served from common serving dishes, with 2 family
members. The dinner lasted 3 hours. The next day, Bob attended a funeral,
hugging family members and others in attendance to express condolences.
Within 4 days, both family members who shared the meal are sick. A third family
member, who hugged Bob at the funeral became sick. But Bob wasn’t done. Bob
attended a birthday party with 9 other people. They hugged and shared food at
the 3 hour party. Seven of those people became ill. Over the next few days Bob
became sick, he was hospitalized, ventilated, and died.
But Bob’s legacy lived on. Three of the people Bob infected at the birthday went
to church, where they sang, passed the tithing dish etc. Members of that church
became sick. In all, Bob was directly responsible for infecting 16 people between
the ages of 5 and 86. Three of those 16 died.
The spread of the virus within the household and back out into the community
through funerals, birthdays, and church gatherings is believed to be responsible
for the broader transmission of COVID-19 in Chicago. (ref)
Sobering right?
Commonality of outbreaks
The reason to highlight these different outbreaks is to show you the commonality
of outbreaks of COVID-19. All these infection events were indoors, with people
closely-spaced, with lots of talking, singing, or yelling. The main sources for
infection are home, workplace, public transport, social gatherings, and
restaurants. This accounts for 90% of all transmission events. In contrast,
infections while shopping appear to be responsible for 3-5% of infections. (ref)
Importantly, of the countries performing contact tracing properly, only a single
outbreak has been reported from an outdoor environment (less than 0.3% of
traced infections). (ref)
So back to the original thought of my post.
Indoor spaces, with limited air exchange or recycled air and lots of people, are
concerning from a transmission standpoint. We know that 60 people in a
volleyball court-sized room (choir) results in massive infections. Same situation
with the restaurant and the call center. Social distancing guidelines don’t hold in
indoor spaces where you spend a lot of time, as people on the opposite side of the
room were infected.
The principle is viral exposure over an extended period of time. In all these cases,
people were exposed to the virus in the air for a prolonged period (hours). Even
if they were 50 feet away (choir or call center), even a low dose of the virus in the
air reaching them, over a sustained period, was enough to cause infection and in
some cases, death.
Social distancing rules are really to protect you with brief exposures or outdoor
exposures. In these situations there is not enough time to achieve the infectious
viral load when you are standing 6 feet apart or where wind and the infinite
outdoor space for viral dilution reduces viral load. The effects of sunlight, heat,
and humidity on viral survival, all serve to minimize the risk to everyone when
When assessing the risk of infection (via respiration) at the grocery store or mall,
you need to consider the volume of the air space (very large), the number of
people (restricted), how long people are spending in the store (workers – all day;
customers – an hour). Taken together, for a person shopping: the low density,
high air volume of the store, along with the restricted time you spend in the
store, means that the opportunity to receive an infectious dose is low. But, for the
store worker, the extended time they spend in the store provides a greater
opportunity to receive the infectious dose and therefore the job becomes more
Basically, as the work closures are loosened, and we start to venture out more,
possibly even resuming in-office activities, you need to look at your environment
and make judgments. How many people are here, how much airflow is there
around me, and how long will I be in this environment. If you are in an open
floorplan office, you really need critically assess the risk (volume, people, and
airflow). If you are in a job that requires face-to-face talking or even worse,
yelling, you need to assess the risk.
If you are sitting in a well ventilated space, with few people, the risk is low.
If I am outside, and I walk past someone, remember it is “dose and time” needed
for infection. You would have to be in their airstream for 5+ minutes for a chance
of infection. While joggers may be releasing more virus due to deep breathing,
remember the exposure time is also less due to their speed.
While I have focused on respiratory exposure here, please don’t forget surfaces.
Those infected respiratory droplets land somewhere. Wash your hands often and
stop touching your face!
As we are allowed to move around our communities more freely and be in
contact with more people in more places more regularly, the risks to
ourselves and our family are significant. Even if you are gung-ho for
reopening and resuming business as usual, do your part and wear a mask
to reduce what you release into the environment. It will help everyone,
including your own business.
Erin S. Bromage, Ph.D., is an Associate Professor of Biology at the University of
Massachusetts Dartmouth. Dr. Bromage graduated from the School of Veterinary
and Biomedical Sciences James Cook University, Australia where his research
focused on the epidemiology of, and immunity to, infectious disease in animals. His
Post-Doctoral training was at the College of William and Mary, Virginia Institute of
Marine Science in the Comparative Immunology Laboratory of late Dr. Stephen
Dr. Bromage’s research focuses on the evolution of the immune system, the
immunological mechanisms responsible for protection from infectious disease, and
the design and use of vaccines to control infectious disease in animals. He also
focuses on designing diagnostic tools to detect biological and chemical threats in
the environment in real-time.
Dr. Bromage joined the Faculty of the University of Massachusetts Dartmouth in
2007 where he teaches courses in Immunology and Infectious disease, including a
course this semester on the Ecology of Infectious Disease which focused on the
emerging SARS-CoV2 outbreak in China