The other day I heard my parents arguing about whether or not it was safe to go to the grocery store:

• Mom: It’s very dangerous. People are dying.
• Dad: It’s completely safe. I wear a mask and keep distance.

In truth, going to the store is neither very dangerous, nor completely safe. It’s somewhere in the middle. But where?

After speaking with my parents for a while I learned that they didn’t have a good mental model on how to think about quantitative risk. This post is for them. This post estimates the probability that an individual contracts COVID before they get a vaccine using rough facts and back-of-the envelope calculations. To be specific, this post uses my parents, who are in their seventies and live in Southern California.

Disclaimer: I am not a medical professional. I encourage you to get actual medical advice from established medical authorities, like the CDC and WHO. I’m just a crank on the internet with a blog.

## End result

Very roughly, I suspect that my parents are about 10% likely to contract COVID before getting vaccinated if they behave like everyone else. I’m probably about 20% likely.

## Reasoning

• Today the US sees around 300,000 cases per day, with a population 1000 times greater, at about 300,000,000

So waking up each day, an American resident has about a 1 in 1000 chance of learning that they have COVID.

If they wake up ten days in a row, they have about at 1 in 100 or 1% chance of learning that they have COVID

• California is a hotspot in the country currently, and LA is a hotspot within California.

Without checking numbers, I’m going to guess that this makes one about 2x more likely to get COVID on a day-by-day basis

• My parents are older (sorry folks) and, like most people their age, have various health issues. They’re likely to be early in line for a vaccine, but not first. Given logistical difficulties I suspect that they’ll get a vaccine some time in March, about 50 days from now

• So 1 in 500 chance of waking up with COVID every day for Californians

x 50 days until vaccination

results in a 1 in 10 chance of contracting COVID prior to receiving a vaccine in March

if they act like their friends and neighbors

As a relatively healthy relatively young person, I’m probably closer to 1 in 5 (20%) due to getting a vaccine a bit later in the year.

## Disclaimer

Remember that I am just a crank on the internet. Please take this post more as an exercise for how to approximate risk, rather than as an actual estimate of that risk.