Tuesday, March 17, 2020

The R⌀ and Why it Matters

What is the coronavirus R0? And how does it help guide as as to what happens next, and where the pandemic is going? On your block, neighborhood, city--and world?

Simply put, R0 is how many people one infected person can infect on average.

But it is a changing and context-dependent number, not a constant.

It's important to understand what it is, and what it is not.

The following is from emcrit.org, bringing "the best evidence-based information from the fields of critical care, resuscitation, and trauma and translate it for bedside use in the Emergency Department (ED) and the Intensive Care Unit (ICU)", dated 3/2/20:

"R⌀ is the average number of people that an infected person transmits the virus to.

"If R⌀ is <1, the epidemic will burn out.

"If R⌀ = 1, then epidemic will continue at a steady pace.

"If R⌀ >1, the epidemic will increase exponentially."

It took me a while to really get it, so give yourself another read to digest this.

And what's the number? As of this month, "current estimates put R⌀ at ~2.5-2.9 (Peng PWH et al, 2/28)."

That means that every person who is infected will infect, on average, 2.5 to 3 more people. That means that the epidemic will increase exponentially.

Remember, it's not a constant. "R⌀ is a reflection of both the virus and also human behavior. Interventions such as social distancing and improved hygiene will decrease R⌀."

Here's what we know: "Control of spread of COVID-19 in China proves that R⌀ is a modifiable number that can be reduced by effective public health interventions."

So R0 changes over time. R0 changes with circumstances. R0 changes with human behavior.

You can influence it. Each and every one of us can.

Here's an example of influence:

"The R⌀ on board the Diamond Princess cruise ship was 15 – illustrating that cramped quarters with inadequate hygiene will increase R⌀ (Rocklov 2/28)."


This means that for each person who was infected in that closed environment, fifteen other people got sick. Makes sense, though: cramped quarters, inadequate hygiene, and people who are already infected. High transmission context.

So the goal is to do the opposite, and that's what all the advice is about, how to lower R0. How?

Social distancing, good hand-washing, disinfectants for inanimate objects like plastic, steel and glass. Staying home when you're sick.

That's how.

Waiting's a poor plan. That just means more cases. Even R0=1 means new people get infected. Maybe you, or someone you care about.

We have to get R0 below 1 to turn the tide, or it's going to be a long, unpleasant trip. You, me, and even them.

Everyone matters.

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