A vaccine when there is no vaccine.

Mark Shrime, MD, PhD
3 min readApr 12, 2020

Two weeks ago, I started wearing a cloth mask in public.

Not going to lie. It feels weird. It’s hot. It’s kinda gross. And when I started wearing it, people gave me wide berth. Boston’s a walking city; we’re used to crowded sidewalks. But my mask guaranteed I’d get six feet of physical distancing—no one wants to walk near a bald, bearded brown guy wearing a mask.

Two days ago, Los Angeles mandated cloth masks for anyone walking outside. Even the President himself, despite his resolute “what’s the big deal” stance toward the pandemic, announced that the CDC recommends everyone to wear a cloth mask outside. (He immediately undercut the CDC by insisting he had no plans to comply).

What’s the big deal with masks? Should we wear them? After all, there isn’t a lot of evidence that wearing a cloth mask protects you from respiratory infections.

Let’s do some more science.

A few weeks ago, I wrote about the dynamics of an epidemic, and specifically about the R0—that infernal number measuring how infectious a pathogen is.

The R0 documents the number of new people, on average, that a single infected person infects. As long as the R0 remains above 1, the epidemic continues. Once it drops below 1, the epidemic dies out. (I’m definitely oversimplifying, but stick with me).

Back then, I’d said that there were three ways for the R0 to drop below 1: we wait for the virus to mutate; we allow the virus to infect the entire population, — killing whomever it kills — until there just aren’t enough susceptible people any more; or we force the R0 downward through social distancing.

Masks play into that last strategy. The R0 depends on two things:

  • The contact rate
  • The transmission probability

The contact rate is pretty simple: how many unique people do I come in contact with in a single day? That’s what social distancing focuses on.

But we can do more. The transmission probability measures how likely it is for the virus to be transmitted if I come in contact with someone. This is where cloth masks come into play.

Because even if they don’t appear to protect me against getting the infection, they do decrease the chances of me spreading the virus to you.



Mark Shrime, MD, PhD

Author, SOLVING FOR WHY | Global surgeon | Decision analyst | Climber | 3x American Ninja Warrior Competitor