Word of the Week: R₀

Each week, Verywell explains a term from health, medicine, science, or technology.

Word of the Week: R₀

How to say it: "R naught" or "R not"

What it means: R₀ is a term that scientists who study how diseases spread (epidemiologists) use when they are talking about how many susceptible people 1 sick person is likely to infect.

Where it comes from: The R₀—or basic reproduction number—was first used by modern scientists in the 1950s. Specifically, by a man named George MacDonald who was studying how malaria spreads.

Where you might see or hear it: When public health experts talk about how viruses like influenza and COVID-19 are spreading, they might refer to the R₀ when they are trying to explain how fast a virus is likely to infect people within a population.

It's important to note that the R₀ indicates the potential for spread among people who have not already been sick with a disease (and would have natural immunity) and have not been vaccinated.

For example, if 1 person has the flu, they will probably get 1 to 2 people who are vulnerable to getting infected. Then, each person that they got sick will probably get another 1 or 2 susceptible people sick, and so on.

Different viruses have different R₀'s. Here are a few examples:

Most flu strains: 1-2
Ebola: 2
Common cold: 2-3
SARS: 2-4
First COVID virus: 2.9
COVID Alpha variant: 4-5
Polio: 5.7
Chickenpox: 10-12
Measles: 12-18

A disease's R₀ is not set in stone. Interventions can help lower the R₀ and get it as close to zero as possible—at which point the spread will stop.

The R₀ shows a range. How many people could get sick depends on how vulnerable they are to infection. For example, people who have immunity are not as likely to get sick and people who do not— and that's one reason why vaccination is so important.

When you might want to use it: If you're talking about COVID, you might bring up R₀ as a way to explain why some experts are concerned that the Omicron variant could make more people sick than the earlier variants of the COVID virus did.

You can also use the R₀ explanation to compare COVID to other contagious illnesses that people are more familiar with, like the flu, colds, and chickenpox.