10 | March | 2021 | The Homa Files

Archive for March 10th, 2021

COVID: Can vaccinated people spread the virus?

March 10, 2021

Logically, the answer is yes, but data is scarce so “the science” is unsettled.
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Yesterday, we posted re: the CDC “guidance” that vaccinated people can hang with other vaccinated people and low risk people (notably children) without masks or social distancing.

See CDC guidance for vaccinated people

That’s good news and very common-sensical and scientifically-based (at least conceptually).

But, what about people who haven’t been vaccinated?

As more people luck out and get vaccinated, the question for those who are still waiting in line for a shot is: “Can we unvaccinated people catch COVID from somebody who has been vaccinated?”

Since there isn’t much post-vaccination history, there isn’t much hard data on the question.

So, we have to think logically and infer the answer.

First, it’s important to keep in mind that the clinical trials for the mRNA vaccines (Pfizer & Moderna), concluded the vaccinations are 90% to 95% effective at preventing symptomatic COVID. The J&J viral vector (DNA) vaccine scores slightly lower at about 70% effectiveness at preventing symptomatic COVID.

Note: The J&J trials were were conducted in a more hostile Covid environment — during a spike in confirmed cases and the emergence of variant Covid strains. So, most experts claim that all 3 vaccines are roughly equivalent in protecting against symptomatic infections.

Most important: Based on their clinical trials, all 3 vaccines are near 100% effective in preventing Covid-related hospitalizations and deaths.

But, the clinical studies did not determine how effective the vaccines are in preventing asymptomatic COVID infections.

They might or might not.

Note: To determine whether the vaccines were effective against asymptomatic COVID would have required frequent periodic COVID testing … which would have been logistically overwhelming … and subject to statistical contamination from false positive (and false negative) test results.

For details on that last point, see:

NY Times: Positive Covid test results misleading: Up to 90% of people testing positive carry inconsequential amounts of the virus

If I test positive for COVID, am I infected? Bayesian math says 2/3s are false positives

Said differently, it is logically possible that vaccinated people catch the virus, but never develop symptoms.

So what?

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Posted in COVID - vaccine | Leave a Comment »

NY Times: Positive Covid test results misleading…

March 10, 2021

Up to 90% of people testing positive carry inconsequential amounts of the virus
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Originally posted 10/16/2020

When COVID case counts surged in July, most “experts” said that — after a time delay — fatalities would surge, too.

They didn’t, causing a lot of head-scratching.

Now the consensus explanation for a statistically significant decline in the infection-to-fatality rate (IFR) is that (1) more asymptomatic people (i.e. minimally effected) were being tested (2) those testing positive were skewed to to younger age groups with IFR rates, and (3) more effective medical treatment and therapeutic drugs were saving more seriously effected patients.

The New York Times has served up a scientific explanation…

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Posted in Covid-19 Testing | Leave a Comment »

If I test positive for COVID, am I infected?

March 10, 2021

The answer may surprise you, and it has big implications for how individuals & organizations respond to positive Covid test results.
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Originally posted 05/27/2020; updated January 17, 2022

In a prior post, we reported that “Asymptomatics” are not rushing to get tested and provided some subjective reasons why that might be (e.g. no doctor referral, high hassle factor, privacy concerns).

OK, let’s up our game a notch or two and throw some math & economics at the problem.

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I’m a fan of “Freakonomics” … the popular call sign for a discipline called Behavioral Economics … the study of the rationality that underlies many seemingly irrational decisions that people sometimes make.

And, in my strategic business analytics course, I used to teach something called Bayesian Inference … a way to calculate probabilities by combining contextual information (called “base rates” or “priors”) with case-specific observations (think: testing or witnessing).

Today, we’ll connect Freakonomics and Bayesian Inference and apply them to the COVID testing situation…

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