How Viruses Spread…

| May 14, 2020 | 0 Comments

I found this information useful as it helps to take away some of the mystery and helps us know what we don't need to be concerned about and what we should be considering. We can't really eliminate all risks and still go about our work and activities. However, we can lessen our risks if we understand how these diseases spread.
Below are some paragraphs from this article. Understand that the numbers are estimates at this point, but they still explain the basic principles.  A link at the end will take you to the complete article.
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The Risks - Know Them - Avoid Them
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?
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 [and singing] 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. Sneezes and 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 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.

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. 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.

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 avoid touching your face!

Erin Bromage | erinbromage.wixsite.com

 

 

Category: Public

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