This post includes excerpts and summarizations of a post by Dr. Erin Bromage which itself went viral with a very large number of views.
Successful Infection = Exposure to Virus x Time
Number of virus particles needed for infection
Additional factors impacting exposure.
Number of virus particles needed for infection
- Based on studies with other coronaviruses experts estimate that as few as 1000 infectious viral particles are all that will be needed
Additional factors impacting exposure.
- Environment
- The more enclosed the space the higher the risk.
- The larger the number of people the higher the risk.
- The way the virus is expelled changes the risk (see the table below.)
- Stage of infection
- 44% of infections are from asymptomatic people.
- Infectious people come in all ages.
Note: I created the following table from information in the article by Dr. Bromage--it was not in the original article.
Where are the biggest personal dangers from reopening?
Any environment that is enclosed, with poor air circulation and high density of people, spells trouble. In studies of major spreading events, all occurred indoors, with people closely spaced, with lots of talking, singing, or yelling.
The main sources for infection are home, workplace, public transport, social gatherings (e.g. weddings, funerals, birthdays), and restaurants. This accounts for 90% of all transmission events. In contrast, outbreaks which spread from shopping appear to be responsible for a smaller percentage of traced infections.
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.
Additional thoughts on different places of exposure
Shopping
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 relatively lower than other indoor activities. 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 riskier.
Indoor spaces with events
Indoor spaces, with limited air exchange or recycled air and lots of people, are concerning from a transmission standpoint. Studies of actual events (summarized briefly below) showed spread of infections during a choir practice (even with social distancing precautions) and in a restaurant, a call center, and a funeral.
Workplaces
If you are in an open floorplan office, you really need to critically assess the risk (volume, people, and airflow). If you are in a job that requires face-to-face talking risks are higher. Ventilation makes a difference.
Outside
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. Please do maintain physical distance, but the risk of infection in these scenarios are low. Here is a great article in Vox that discusses the low risk of running and cycling in detail.
Conclusion
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.
Brief overview of studies of specific spreading events
Any environment that is enclosed, with poor air circulation and high density of people, spells trouble. In studies of major spreading events, all occurred indoors, with people closely spaced, with lots of talking, singing, or yelling.
The main sources for infection are home, workplace, public transport, social gatherings (e.g. weddings, funerals, birthdays), and restaurants. This accounts for 90% of all transmission events. In contrast, outbreaks which spread from shopping appear to be responsible for a smaller percentage of traced infections.
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.
Additional thoughts on different places of exposure
Shopping
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 relatively lower than other indoor activities. 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 riskier.
Indoor spaces with events
Indoor spaces, with limited air exchange or recycled air and lots of people, are concerning from a transmission standpoint. Studies of actual events (summarized briefly below) showed spread of infections during a choir practice (even with social distancing precautions) and in a restaurant, a call center, and a funeral.
Workplaces
If you are in an open floorplan office, you really need to critically assess the risk (volume, people, and airflow). If you are in a job that requires face-to-face talking risks are higher. Ventilation makes a difference.
- Example: In meatpacking plants workers are densely packed, communicating closely in a cold room (coldness is virus-preserving). There are now outbreaks in 115 facilities across 23 states, 5000+ workers infected, with 20 dead.
Outside
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. Please do maintain physical distance, but the risk of infection in these scenarios are low. Here is a great article in Vox that discusses the low risk of running and cycling in detail.
Conclusion
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.
Brief overview of studies of specific spreading events
- Restaurant: An asymptomatic infected person sat at a table with 9 friends for 1 to 1.5 hours. Approximately 50% of the people at the infected person's table became sick over the next 7 days. 75% of the people at an adjacent downwind table became infected. And even 2 of the 7 people at an upwind table were infected—probably due to air turbulence
- Call center: A single infected employee came to work on the 11th floor of a building with 216 employees. Over the period of a week, 94 of those people became infected (43.5%) of whom 92 became sick. Another 3 people on other floors of the building were infected. This highlights the importance of exposure and time in the spreading of SARS-CoV2.
- Choir practice: 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 told 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. Over a period of 4 days, 45 of the 60 choir members developed symptoms and 2 died. The youngest infected was 31 but they averaged 67 years old.
- Indoor sporting event: From a curling event with 72 attendees 24 of the 72 became infected.
- Spreading from one person who attended a funeral then a birthday party: A person shared a takeout meal served from common serving dishes with 2 family members. The dinner lasted 3 hours. The next day, the person attended a funeral, hugging family members and others in attendance to express condolences. Within 4 days, both family members who shared the meal became sick. A third family member, who hugged the original person at the funeral became sick. The original spreading person then attended a birthday party with 9 other people the next day over 3 hours. Seven of those people became ill. Over the next few days, the original person became sick, was hospitalized, ventilated, and died.
- Three of the people infected at the birthday went to church, where they sang, passed the tithing dish etc. infecting 16 people between the ages of 5 and 86. Three of those 16 died.
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