A 3-Minute Crash Course in Coronavirus Transmission

A 3-Minute Crash Course in Coronavirus Transmission

Last year my girlfriend threw me an amazing birthday party. After an elaborate, month-long build-up, she lured me through a series of challenges in Central Park on the day of my birthday. When I finally completed my quest, finding the keys to rescue my fair maiden trapped in a tall tower, we had a celebratory feast in the castle courtyard. Costumes were mandatory. Some even wore masks. We ate outdoors not because we had to, but because the setting and novelty made the added effort worth it.

How the world has changed.

A few weeks ago I went to a birthday celebration in Central Park. It was a socially-distanced tea party, and it was the first time in months that I had seen the non-pixellated versions of most of my friends. My eyes feasted upon their diversely-masked faces, scattered amongst a grassy field checkered with spaced-out picnic blankets. In the center was a delicious spread of homemade cupcakes and serving-sized bottles of tea and water. It was great to see everyone, but an undercurrent of uncertainty ran through the festive atmosphere. Was what we were doing socially acceptable? Were we putting ourselves or others at risk? Were we allowed to eat and talk at the same time? If so, what did we do with our masks?

In an ideal world, each individual would base their behavior on two factors: a shared understanding of the facts, and their own level of risk tolerance. Practically speaking, there is a third factor that trumps the other two: social conformity. Regardless of the facts, we tend to act in a manner that will earn us the respect of our peers. This holds even when it comes at the cost of our health (e.g. social drinking, eating cake at birthday parties, dangerous dares, etc.).

We need to make our desire for social conformity push us in the direction that best maximizes comfort and minimizes risk. To do this, we need a shared model of disease transmission that can quickly and accurately be applied to a diverse range of scenarios.

It starts with the facts.

3-Minute Crash Course Starts Here

Live virus can be found in the respiratory tract, urine, and feces. It is typically spread via large droplets released when sneezing or coughing, or by aerosolized particles–droplets of moisture that are small enough to remain floating in the air for hours. Small amounts of aerosols are released when breathing, medium amounts when speaking or breathing heavily, large amounts when speaking loudly or singing, and even larger amounts when coughing. It can also be spread through aerosols released when a toilet is flushed. Infection happens through the mucosal membranes in the nose, throat, and eyes. Virus particles can reach these membranes either via inhalation or from being carried by contaminated hands.

Viral infection operates on a sliding scale for both the infector and infectee. The number of live virus particles in respiratory droplets or aerosols is proportional to the viral load in your body–how much the virus has already replicated. Viral load cannot be inferred from the presence of symptoms or their severity. It usually peaks in the first week after symptom onset, but can be at infectious levels several days before symptoms begin to show. To become infected, the Accumulated Exposure must reach an Infectious Dose of live virus particles in a short enough window of time. This infectious dose varies from person to person based on their immune system health. Rodent studies and expert opinions estimate that the infectious dose could be as low as 500 SARS-CoV2 infectious viral particles. If the Accumulated Exposure is much higher than the Infectious Dose it could lead to a more severe infection. This has been proven to be the case with SARS, MERS, and influenza, as well as initial animal studies with COVID-19.

A single sneeze or cough can deliver an infectious dose. Several minutes of conversation could do the same. Louder conversation is riskier. Singing even more so. The longer the exposure, the greater the accumulated dose, and the higher the risk.

A 2 meter / 6 feet distance is not by itself sufficient to eliminate risk. The cloud of particles from a sneeze can travel up to 7-8 meters or 26 feet.

Infectious particles can be blocked both by physical barriers and by environmental factors.

Masks are better for blocking large droplets than they are for aerosols. Well-fitted N95 respirators can block 95% of aerosols. Surgical masks can provide a 2-3x reduction in viral copy numbers. Cloth masks range from 26-98% depending on the fit, fabric, and number of layers. The key point to remember is that even if a mask blocks 50% of particles, that halves the total accumulated exposure. Both parties wearing masks that block 80% of particles lowers the accumulated exposure by a factor of 25.

There is very little evidence of outdoor transmission of the coronavirus. This is mainly due to the large amount of airflow dispersing particles extremely quickly. Heat, ultraviolet light, and humidity all help to either keep the particles from spreading or kill the virus in-transit. However, unlikely is not impossible. One study of 7,324 confirmed cases identified only a single outdoor outbreak with 2 cases, involving a long conversation at close proximity. The closer we can make our indoor spaces resemble our outdoor ones, the less we’ll have to worry about viral transmission.

If we can learn to properly adjust our behavior, protective equipment, and environmental controls, we will be able to go back to our old lives and spaces without fear.

Using Facts to Guide Behavior

Now that we have a basic understanding of how the disease is transmitted, we can build a model to determine future behavior. After months of quarantine, many are unsure how to go about re-introducing social activity back into their lives. Some err on the side of being overly cautious, still unused to human contact. In contrast, others throw caution to the wind and host house parties. Still others let their political affiliations guide their behavior rather than science. The time has come to shift to a more rational approach.

To properly evaluate each new potential form of human contact, we must use three simple rules:

  1. Keep total Accumulated Exposure below an Infectious Dose
  2. Total Accumulated Exposure = Exposure Level x Time
  3. Risk of being near an infected person ≈ % of Active (generally asymptomatic) Cases in region x # of People Present

Six months into the pandemic, the exact numbers for each aspect of the equation are still somewhat uncertain. The level of situational and individual variance means that they will likely never be fully known. Nonetheless, with a proper understanding of transmission dynamics, we can use a qualitative analysis to inform our decisions.

Real-Life Examples of Coronavirus Risk Evaluation

For each of these evaluations, rule #3 above also applies: risk increases with more people around and higher disease prevalence in the area. This could be modified further by what you know about the exposure levels for the people involved.

Walking alone outdoors: Excellent ventilation, sun, medium-to-large distances, and very short exposure times = almost no chance of reaching an infectious dose, even without a mask, unless you are sneezed or coughed on.

Conversation outdoors: Excellent ventilation, sun, short-to-medium distances, and medium-to-long exposure times =minimal risk, but best to maintain distance and/or masks. Better to sit side by side than facing each other, so as to direct the aerosol-cloud away.

Barbecue outdoors: Excellent ventilation, sun, short distances, medium-to-long exposure times, shared contact of items, and frequent touching of mouths = medium-to-high risk. Best to bring your own food and wear masks when not actively eating.

Riding the Subway: Poor ventilation, minimal environmental risk reduction, distances vary based on crowding levels, medium exposure times = medium-high risk during crowded times. Low-medium risk during non-crowded times. Very low risk during non-crowded times if everyone is wearing masks and hands are washed immediately after exiting (if handholds are touched).

Sharing a Car Ride: Terrible ventilation, no environmental risk reduction, short distances, long exposure times = very high risk. Best to all wear masks (respirators if possible), sit diagonally across from the driver, and keep windows open when possible–or keep the AC set to “Fresh Air Mode” when on the highway. This will cause it to bring air in from the outside rather than recirculate the same potentially infected air.

SOURCE: Jayaweera M, Perera H, Gunawardana B, Manatunge J. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy [published online ahead of print, 2020 Jun 13]. Environ Res. 2020;188:109819. doi:10.1016/j.envres.2020.109819

Indoor Dinner Party: Poor ventilation, close distances, shared food, shared bathroom, impossible to keep masks on, very long exposure times = extremely high risk.

These examples are only a small sample of the types of situations you might be finding yourself in these days. It would be impossible to cover them all, but hopefully, this will give you a framework upon which to make decisions going forward.

If you have a particularly interesting situation that you are trying to evaluate, feel free to let me know in the comments and I’ll try to give you my two cents.


Note: The information in this article is based on the current snapshot of scientific research regarding coronavirus transmission. As more information becomes available, some aspects of the model could shift. For the sake of brevity, each point made in the summary linked to a single supporting document. Feel free to contact me for a more complete set of sources on a given topic.

For a closer look at how masking has worked out on a population level, see the previous article in this series.

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