## Covid19 Exposure Risk Calculator

### How likely is it that among N people, at least one is Covid+?

This is a question you probably asked yourself: how likely is it for someone to be covid positive at a wedding? What’s the probability of a child being positive at school? Thanks to this calculator, you can now quantify it.

In the “incidence” box, write the virus incidence for the place and age range you want to assess. In the “number of people” box, write how many people you expect to attend the event in question.
All you have to do now is running the calculator!

Incidence: /100000

Number of people:

Risk that at leat one people is Covid:

### How does it work?

Each person has a probability “p” to be covid positive.

Let’s imagine that 4 people meet. We can calculate how likely it is for none of them to be positive:

This probability « p » can be estimated thanks to the incidence : an incidence of 1 000 means that among 100 000 people, 1 000 are positive ; in other words, that 1% of them are positive. We can now say that p = incidence / 100 000. Note that this probability is vey likely to be under-estimated. As the logical event « at least one person is positive” is the contrary of the event “among all people, none is positive”, we can guess how likely it is that at least one person is Covid + : 1 – ( 1 – i / 100 000)^N, With « i » the incidence and « N », the number of people.

Authors: Elias Orphelin, Guillaume Rozier