For Coronavirus, the Name of the Game Is Minimizing the Probability of Infection.

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  • 03/02/2023

It’s clear from what people are posting on social media, and the questions they’ve submitted to Town Hall Q&As, that many are still unclear as to how the coronavirus that causes COVID-19 is transmitted, and what can be done to avoid it. The operative concept can be summarized in a single phrase: minimize the probability.

Each of these measures lowers the probability that an infectious dose of virus will find its way to your respiratory tract.

Let’s take a weather-related example as an analogy. Say it’s sleeting, the temperature is 30 degrees, and the precipitation is forming a solid ice sheet as it hits the roadway. These are prime conditions for skidding and an accident, but there are a number of things you can do to lessen the probability of that occurring: putting chains on your tires, choosing your four-wheel-drive SUV instead of the Ferrari, driving at a crawl, etc. Of course, you can lower the risk to zero by staying home entirely.

The adaptations needed to avoid coronavirus infection are analogous—wear a mask (and possibly a face shield in addition), shelter in place, avoid crowded indoor spaces, practice social distancing, don’t touch your face, and wash your hands frequently and thoroughly. Each of these measures lowers the probability that an infectious dose of virus will find its way to your respiratory tract. (There are, however, other factors that affect the probability of infection that you can’t control, such as your age, whether you’re immunocompromised, or have other comorbidities.)

It sounds like a simple enough concept, but it seems to elude some people who should be able to grasp it. At a June 26th House of Representatives hearing of the Select Subcommittee on the Coronavirus Crisis, Rep. Jamie Raskin (D-MD) chastised Republican members who were not wearing masks. Republicans, several of whom had worn masks into the hearing room before removing them, remonstrated that they could practice social distancing safely while seated maskless. “We are six feet apart. We don’t need a mask,” said Rep. Mark Green (R-TN), who is a physician.

With all due respect to Congressman Dr. Green, he doesn't get it. Six feet of distancing isn’t magic; it’s just a guess about what manageable distance between individuals lowers the probability of transmission to a reasonable level. And even then, we know that when a person shouts, sings, coughs, or sneezes, droplets or aerosols (which contain minuscule particles and stay airborne for long periods) can travel much farther than six feet.

There are plenty of interventions that can lower the probability of contracting COVID-19, even to near zero. This is the essence of preventive medicine in general, and epidemiology in particular. For example, many of us take drugs to lower our cholesterol levels or blood pressure to reduce the likelihood of cardiovascular disease. Analogously, there is intense competition to develop a vaccine to prevent COVID-19, which would be a game-changer.

In all these cases, what’s important to note is our strategy is one of managing probability—more specifically, reducing the chances of your being exposed to an amount of coronavirus sufficient to penetrate your body’s natural defenses and cause an infection. Thus, the more preventative measures, the better.

[caption id="attachment_182687" align="aligncenter" width="1920"]Healthcare workers. Healthcare workers.[/caption]


I was asked recently by a friend about the transmissibility of the virus from dog fur—for example, if your dog was petted by a virus-shedding stranger you encountered during a walk. The CDC’s thinking as of June 22 was that the probability of that kind of transfer is very low, although a few weeks earlier, their specific advice about pets had been, "Treat pets as you would other human family members—do not let pets interact with people outside the household."

Those somewhat contradictory statements reflect that the science of viral transmission is all about probabilities.

Those somewhat contradictory statements reflect that the science of viral transmission is all about probabilities. Just as the chance of your being infected by another person is a function of time of exposure, the number of virus particles you encounter, whether there's dilution (lowering of the concentration of virus particles) from good ventilation and filtration—and a few other factors—the same is true of transfer of viral particles from a pet. 

So why the earlier, more risk-averse statement from the CDC? The most likely scenario for transmission would be that person A, who is infected with coronavirus and in the high-shedding stage (just before or just as he becomes symptomatic) breathes, coughs, or sneezes on a dog, and then shortly afterward, person B pets the dog, picks up a significant dose of virus, and then unwittingly touches his face, getting enough virus into his mouth, nose, or eyes to become infected. 

In other words, all of those things would have to happen, keeping in mind that the virus is rapidly killed by sunlight and diluted by good ventilation, including wind. (Not to mention that the body’s nonspecific first line of defense, “innate immunity,” can fend off small amounts of pathogens, including coronavirus.) Therefore, although not zero, the probability of becoming infected in such a scenario is surely very low.

As the pandemic continues, we’re learning more about risk and probability of transmission all the time. For instance, in a preprint of a journal article published on June 20th, academic researchers reported that a few “superspreaders” of coronavirus infections are disproportionately responsible for the spread of disease: just 2% of infected people were responsible for 20% of transmissions. At this point, we still don’t know what makes people superspreaders and how to detect them, or under what conditions they spread the virus so effectively. A better understanding of these phenomena would help us to reduce the probability of transmission.

In the meantime, the Texas Medical Association has just released a chart that ranks the risk --i.e., probability of contracting COVID-19--from various behaviors and activities:

[caption id="attachment_182690" align="aligncenter" width="819"]Credit: Texas Medical Association. Credit: Texas Medical Association.[/caption]

Academic and industrial researchers are working feverishly on every imaginable aspect of how to prevent and treat COVID-19 infections—from epidemiology, virology, immunology, and pulmonary medicine to the development of drugs and vaccines. And it is already bearing fruit! In addition to learning a good deal about transmission of infections, physicians have become much more proficient at managing COVID-19 patients in ICUs, and the mortality rate has been dropping steadily. Hundreds of drugs, and more than 150 vaccines, are now in various stages of development, and 29 vaccines are in clinical trials. But the generation of critical new knowledge and products takes time. As my old boss, FDA Commissioner Dr. Frank Young used to say, “Walking on water wasn’t built in a day.” (He was prone to mixing his metaphors.)

For the time being, masks, social distancing, and frequent hand-washing are important; these measures disrupt the chain of events necessary for infection to occur, lowering its probability.

There’s a high probability that’s all you need to know.

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