Many years ago, a Dan O'Bannon script rumored to be in consideration for Alien^3 circulated on the Interwebs. O'Bannon's tale had teh Alien arriving on Earth in a form of a virus, one that converts the host directly sans a facehugger instar. In what was sure to have become an infamous scene, a Weyland-Yutani bean counter is PowerPointing to a crowded boardroom, coughs once or twice, then a fully-formed adult Alien bursts from his shuffled mortal coil as it splats to the carpet in a bloody heap.
Until that moment, the bean counter had no idea something was amiss. It's a strategem designed to achieve the viral gold standard of success: transmission. That is what a virus lives for (or doesn't live for, as it were).
Perfect organism, Ash called it.
Which brings us to COVID-19.
If you are a virus, your goal is to replicate. This is primarily a question of evading detection. At the cellular level, you must literally evade detection of the host's immune system. A pathogen hopes to do so by expressing surface markers that don't trigger a response. It's akin to tucking your mayhem device someplace the TSA won't palpate when they feel you up at the airport.
That's swell for today, but to go pro a virus needs to get its progeny into another host and preferably into many. Exponential curves notwithstanding, transmission is actually a rather tricky business. To leap from one moist pinky to another, the pinkys must sufficiently mingle. Viruses may float but they don't fly. They also lurk, but once outside a host their hours are numbered.
As such, your reign can be effectively cut short if the hu-mans wise to your presence. All they need do is separate the already-infecteds from the not-yets. This critically depends on the ability to identify the infected before they infect others.
Which is largely up to the virus. Most infections produce symptoms somebody will notice. The more severe the symptoms, the easier the detection. Ebola, for example, is harder to spread than the common cold because the symptoms of Ebola are so horrific. A global pandemic of Ebola is unlikely precisely for this reason. On the other hand, HIV can remain undetected for years, which is how AIDS got so firmly entrenched in its target communities.
Yet, an absence of symptoms is not the whole story. The host must be able to pass on the pathogen during the asymptomatic period. This is a problem to some extent with all infectious diseases, one that often wreaks havoc on containment efforts.
As we now know, COVID-19 has a significant asymptomatic infectious period. I wondered how such a feature alters the dynamics of an epidemic. Professionals might explore the matter using an SEIR model, in which exposed is added to the susceptible, infected, and recovered classes. As it turns out, there's a simpler option for us amateur epidemiologists.
I posted a Javascript some weeks back that allowed you to simulate the global spread of COVID-19. During coding, I discovered a peculiar behavior: No matter what, COVID always emerged pretty much everywhere eventually.
I traced this quirk to floating point arithmetic. The number of infecteds in the sim is represented as a proportion of the population. However, real diseases traffic in numbers of infected -- an integer. The solution to obtaining more realistic dynamics was to threshold the proportion at the end of each sim step, implicitly adding a density threshold for transmission (something like an Allee effect).
As such, including the effects of an asymptomatic infectious period is easy: We simply don't threshold the infections. The modified version of the sim is provided below:
(hard) --- transmission --- (easy)
Footnote: I occasional find Chrome won't run this. If you experience wonkyness, try reloading the page.
Operation is the same as before: choose a slider value, click somewhere in the map to start the infection, close or don't close the airports, observe the results. What you'll discover is it's all but impossible to prevent a global pandemic. Even if it starts in a remote location. Even if you isolate long before cases appear. By the time it's apparent the disease has arrived, it's too late. The disease was there already.
These results jive with what we're seeing in the news. In the US, states that have reopened are experiencing a resurgence in COVID-19 cases. States that remain in lock down have not. It's a consequence of an asymptomatic infectious period on the spatiotemporal dynamics.
All of which underscores the importance of individual efforts to break transmission. Distancing and masks aren't liberal plots, they're pleas of public health professionals desperate to keep civilization from going over the cliff.
COVID wants to spread. It will use you to do so. It doesn't care who you are. It doesn't care where you live. It doesn't care who you voted for or what flag you fly. As far as COVID is concerned, you are just meat.
And it already has you surrounded. Act accordingly.
Until that moment, the bean counter had no idea something was amiss. It's a strategem designed to achieve the viral gold standard of success: transmission. That is what a virus lives for (or doesn't live for, as it were).
Perfect organism, Ash called it.
Which brings us to COVID-19.
If you are a virus, your goal is to replicate. This is primarily a question of evading detection. At the cellular level, you must literally evade detection of the host's immune system. A pathogen hopes to do so by expressing surface markers that don't trigger a response. It's akin to tucking your mayhem device someplace the TSA won't palpate when they feel you up at the airport.
That's swell for today, but to go pro a virus needs to get its progeny into another host and preferably into many. Exponential curves notwithstanding, transmission is actually a rather tricky business. To leap from one moist pinky to another, the pinkys must sufficiently mingle. Viruses may float but they don't fly. They also lurk, but once outside a host their hours are numbered.
As such, your reign can be effectively cut short if the hu-mans wise to your presence. All they need do is separate the already-infecteds from the not-yets. This critically depends on the ability to identify the infected before they infect others.
Which is largely up to the virus. Most infections produce symptoms somebody will notice. The more severe the symptoms, the easier the detection. Ebola, for example, is harder to spread than the common cold because the symptoms of Ebola are so horrific. A global pandemic of Ebola is unlikely precisely for this reason. On the other hand, HIV can remain undetected for years, which is how AIDS got so firmly entrenched in its target communities.
Yet, an absence of symptoms is not the whole story. The host must be able to pass on the pathogen during the asymptomatic period. This is a problem to some extent with all infectious diseases, one that often wreaks havoc on containment efforts.
As we now know, COVID-19 has a significant asymptomatic infectious period. I wondered how such a feature alters the dynamics of an epidemic. Professionals might explore the matter using an SEIR model, in which exposed is added to the susceptible, infected, and recovered classes. As it turns out, there's a simpler option for us amateur epidemiologists.
I posted a Javascript some weeks back that allowed you to simulate the global spread of COVID-19. During coding, I discovered a peculiar behavior: No matter what, COVID always emerged pretty much everywhere eventually.
I traced this quirk to floating point arithmetic. The number of infecteds in the sim is represented as a proportion of the population. However, real diseases traffic in numbers of infected -- an integer. The solution to obtaining more realistic dynamics was to threshold the proportion at the end of each sim step, implicitly adding a density threshold for transmission (something like an Allee effect).
As such, including the effects of an asymptomatic infectious period is easy: We simply don't threshold the infections. The modified version of the sim is provided below:
---
(hard) --- transmission --- (easy)
Operation is the same as before: choose a slider value, click somewhere in the map to start the infection, close or don't close the airports, observe the results. What you'll discover is it's all but impossible to prevent a global pandemic. Even if it starts in a remote location. Even if you isolate long before cases appear. By the time it's apparent the disease has arrived, it's too late. The disease was there already.
These results jive with what we're seeing in the news. In the US, states that have reopened are experiencing a resurgence in COVID-19 cases. States that remain in lock down have not. It's a consequence of an asymptomatic infectious period on the spatiotemporal dynamics.
All of which underscores the importance of individual efforts to break transmission. Distancing and masks aren't liberal plots, they're pleas of public health professionals desperate to keep civilization from going over the cliff.
COVID wants to spread. It will use you to do so. It doesn't care who you are. It doesn't care where you live. It doesn't care who you voted for or what flag you fly. As far as COVID is concerned, you are just meat.
And it already has you surrounded. Act accordingly.
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