Still more about the NY antibody test results…

What about the 3% of New Yorkers floating around while infected but asymptomatic?
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In a prior post, we squeezed the NY antibody test results pretty hard and estimated that about 600,000 New Yorkers are walking around at any one time infected with the coronavirus but exhibiting no or very mild symptoms. That means that about 3% of NY’s population are asymptomatic “hidden carriers” who may be unknowingly spreading the disease.

To understand their significance …

Most infectious disease epidemiology models are built on the “SEIR” construct: how many people are susceptible to a virus … of them, how many are likely to get exposed to it … of them, how many are likely become infected … and of them, how many are likely to recover, perhaps with some degree of immunity. The modelers then calibrate a virus’s behavior, estimating how long it takes people to move from susceptible to exposed to infected to final resolution (recovery or death).

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My former strategy students should recognize the SEIR construct as a basic hierarchy-of-effects model, similar in design to, say, the classic marketing awareness – trial – repurchase model.

And, the spread effects are a classic Bass Diffusion Model application with infected people playing the role of “innovators” and susceptible people playing the role of “imitators”.

Let’s dive a little deeper…

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Working through the SEIR Model, since we are all susceptible to the coronavirus, the big questions are (1) how likely we are to get exposed to it and (2) what’s the likelihood that, when exposed, we get infected.

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I think that I’m OK, but I’m not so sure about you

Obviously, the more we come in close contact with infectious carriers, the higher our risk.

The corona virus is unique in that the majority of disease carriers are asymptomatic – they’re unrecognizable “hidden carriers” who are impossible to identify (i.e. even they don’t know they’re infected) and, thus, tough to avoid.

So, the likelihood of bumping into into one of them is a function of how many of them there are circulating in our local area.

If these asymptomatic carriers are a high proportion of the population, our risks are high. If there are only a few of them, our exposure risk is minimal.

So, wouldn’t you like to know your odds – i.e. know how many of these hidden virus carriers there are in your neighborhood? I sure would.

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Again, in NY state, we estimate that roughly 3 in every 100 people are carrying the virus at any one time.

That may sound like a small number – long odds. But, don’t be lulled into a false sense of confidence.

A recent Gallup survey found that, on average, people make close contact with 10 people each day.

If 3% of your local population are hidden carriers, what’s the likelihood that at least 1 of 10 daily contacts is a hidden carrier?

Answer: It’s not 3% … it’s a whopping 26%

To solve the probability problem, you have to flip the probability question around and ask “What is the likelihood that none of the 10 are infected?”

If there’s a 3% chance that any one of the 10 are infected, there’s a 97% chance that they’re not infected.

So, the chance that all 10 are infection-free is 97% raised to the 10th power … that’s a 74% probability that none of the 10 are carriers. That leaves a probability of 26% that at least 1 is infected.

Takeaway: If asymptomatic carriers are 3% of your local population and you have contact with 10 people each day, there’s a 26% chance, each day, that’s you’ll have contact with somebody who’s infected.

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Beware the unmasked

Of course, being exposed to a person infected with the virus doesn’t necessarily mean that you’ll be infected.

That’s where social distancing and masking comes into play … and minimize the contact risk.

I’d like to know how risky my local area is (that’s called “surveillance testing”) … but, right now, we’re flying blind.

So, until we know that there aren’t many asymptomatic carriers floating around, let’s hedge our bets: stay away from anybody who isn’t wearing a mask.

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