COVID: Can vaccinated people spread the virus?

Logically, the answer is yes, but data is scarce so “the science” is unsettled.

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?


The pivotal question is: “Can COVID infectees who are asymptomatic spread the virus?

There is some “science” on that question.

Based on some modeling work, the CDC has concluded that almost 60% of all COVID infections were transmitted from people with no evident symptoms. Source: JAMA

Prior studies have ranged the number between 35% and 45%.

Though the numbers vary a bit, the fundamental conclusion stays the same: Asymptomatic infectees are responsible for a statistically significant (near majority) portion of COVID transmissions.


So, connecting the dots, somebody who has been vaccinated may be asymptomatically infected with COVID … and, since many (most?) infections are transmitted from infectees who are  asymptomatic, Pascal’s Wager applies:

> If you assume that vaccinated people may be asymptomatically infected and take the usual precautions (masks & social distancing), then the  vaccinated people may make fun of you for being paranoid (a small downside), but you’re unlikely to get infected, which is a big upside.

> On the other hand, if you assume that vaccinated people are virus-free and you let your guard down then there’s a small upside (e.g. the convenience of no masks and allowable hugging) … but, if they turn out to be asymptomatically infected … then there’s a statistically significant chance that you’ll get infected, which is a huge downside.

Place your bet.

DISCLAIMER: I’m not a medical professional or scientist — just a curious, self-interested guy.  So, don’t take anything that I say or write as medical advice. Get that from your doctor!


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