How to actually catch Covid-19? By now we know: most people got infected in their own home. One household member contracted the Coronavirus somewhere outside and brought it home where sustained contact between household members did lead to infections. But where are people contracting the infection in the first place?
People are worrying about grocery stores, bike rides, inconsiderate runners, folks who are not wearing masks … is all this the major concern? Well, not really. Let’s explain.
The infectious dose
In order to get infected you need to get exposed to a certain infectious dose of the virus. Based on infectious dose studies with other coronaviruses, it appears that small doses may be needed for an infection to take hold in your body. Some experts estimate that as few as 1,000 infectious particles are all it needs. Infection with 1,000 particles could occur in just one eye-rub or one breath, containing these 1,000 particles. Or 100 infectious particles are inhaled with each breath over 10 breaths, or 10 particles are inhaled with 100 breaths. Each of these situations can lead to an infection.
How much Covid19 is released into the environment?
1: Bathroom | Bathrooms have a lot of high touch surfaces, door handles, faucets, stall doors. Transfer risk in this environment can be high. Toilet flushing does aerosolize many droplets.
2: Cough | A single cough releases approx. 3,000 droplets. Most droplets are large, and fall to the ground quickly, but many smaller droplets stay in the air and travel across a room.
3: Sneeze | A single sneeze releases about 30,000 droplets. Most droplets from Sneezing are small and travel great distances. If a person is infected, the droplets in a single cough or sneeze may contain as many as 200,000,000 Covid19 particles which can all be dispersed into the environment around them.
4: Breath | A single breath releases 50 to 5,000 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 Covid19 yet, but we can use Influenza as a guide. Studies have shown that a person infected with Influenza releases up to 33 infectious viral particles per minute.
Let’s imagine, you are already infected, but you don’t know it yet. Anyone you spend greater than 10 minutes with in a face-to-face situation is potentially infected. Anyone who shares a space with you (office, mosque, school, home) for an extended period is potentially infected. This is 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.
As we move back to work, or go to a restaurant, let’s look at what can happen in those environments.
Some really great shoe-leather epidemiology demonstrated clearly the effect of a single asymptomatic carrier in a restaurant environment (see infographic). The infected person at seat A1 sat at a table and had dinner with nine friends. Dinner took about 1 to 1.5 hours. During this meal, the virus carrier released low-levels of virus into the air from 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 seven days. In addition, 75% of the people on the adjacent downwind table became infected. And even two of the seven 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.
Another great example is the outbreak in a call center in South Korea. A single infected employee came to work on the 11th floor of a building in Seoul. That floor had 216 employees. Over the period of a week, 94 of those people became infected (43.5%: the blue chairs). 92 of those 94 people became sick (only two remained asymptomatic). Notice how one side of the office is primarily infected, while there are very few people infected on the other side. While exact number of people infected by respiratory droplets / respiratory exposure versus fomite transmission (door handles, shared water coolers, elevator buttons etc.) is unknown. It serves to highlight that being in an enclosed space, sharing the same air for a prolonged period increases your chances of exposure and infection. Another three people on other floors of the building were infected, but the authors were not able to trace the infection to the primary cluster on the 11th floor. Interestingly, even though there were considerable interaction between workers on different floors of the building in elevators and the lobby, the outbreak was mostly limited to a single floor. This highlights the importance of exposure and time in the spreading of Covid19.
The community choir in Washington State, USA. 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. They even went to the lengths to tell choir members prior to practice that anyone experiencing symptoms should stay home. A single asymptomatic carrier infected most of the people in attendance. The choir sang for 2 1/2 hours, inside an enclosed rehearsal hall 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.
A super spreading event occurred during a curling event in Canada. The 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.
Just to see how simple infection-chains can be, this is a real story from Chicago, USA. Only the name used here is fake.
Bob was infected but didn’t know. Bob shared a takeout meal, served from common serving dishes, with two family members. The dinner lasted three hours. The next day, Bob attended a funeral, hugging family members and others in attendance to express condolences. Within four 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 nine other people. They hugged and shared food at the three hour party. Seven of those people became ill.
And Bob’s transmission chain still wasn’t done. Three of the people Bob infected at the birthday went to church. Members of that church became sick. In all, Bob was directly responsible for infecting 16 people between the ages of 5 and 86. Three died.
Indoor vs Outdoor
The spread of the virus within the household and back out into the community through funerals, birthdays, and religious gatherings is believed to be responsible for the broader transmission of COVID-19.
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, gatherings, restaurants. This accounts for 90% of all transmission events.
In contrast, outbreaks spread from shopping appear to be responsible for a small percentage of traced infections. And only one single outbreak has been reported from an outdoor environment (= less than 0.3% of traced infections).
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