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:R₀is a term that scientists who study how diseases spread (epidemiologists) use when they are talking abouthow many susceptible people 1 sick person is likely to infect.
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 getanother1 or 2 susceptible people sick, and so on.
Different viruses have different R₀'s. Here are a few examples:
Most flu strains:1-2Ebola:2Common cold:2-3SARS:2-4First COVID virus:2.9COVID Alpha variant:4-5Polio:5.7Chickenpox:10-12Measles:12-18
A disease’s R₀ is not set in stone.Interventionscan 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 whyvaccinationis 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 earliervariants of the COVID virusdid.
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.
What Do We Know About the Omicron Variant So Far?
SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Popular Science.Just how contagious is COVID-19? This chart puts it in perspective. February 20, 2020.University of Michigan.How Scientists Quantify the Intensity of an Outbreak Like COVID-19. March 17, 2020.Vox.Why omicron could bring the deadliest phase in the pandemic yet. December 19, 2021.
Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Popular Science.Just how contagious is COVID-19? This chart puts it in perspective. February 20, 2020.University of Michigan.How Scientists Quantify the Intensity of an Outbreak Like COVID-19. March 17, 2020.Vox.Why omicron could bring the deadliest phase in the pandemic yet. December 19, 2021.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Popular Science.Just how contagious is COVID-19? This chart puts it in perspective. February 20, 2020.University of Michigan.How Scientists Quantify the Intensity of an Outbreak Like COVID-19. March 17, 2020.Vox.Why omicron could bring the deadliest phase in the pandemic yet. December 19, 2021.
Popular Science.Just how contagious is COVID-19? This chart puts it in perspective. February 20, 2020.
University of Michigan.How Scientists Quantify the Intensity of an Outbreak Like COVID-19. March 17, 2020.
Vox.Why omicron could bring the deadliest phase in the pandemic yet. December 19, 2021.
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