NEWS Health News Word of the Week: R₀ By Team Verywell Health Published on December 21, 2021 Fact checked Verywell Health content is rigorously reviewed by a team of qualified and experienced fact checkers. Fact checkers review articles for factual accuracy, relevance, and timeliness. We rely on the most current and reputable sources, which are cited in the text and listed at the bottom of each article. Content is fact checked after it has been edited and before publication. Learn more. by Angela Underwood Fact checked by Angela Underwood LinkedIn Angela Underwood's extensive local, state, and federal healthcare and environmental news coverage includes 911 first-responder compensation policy to the Ciba-Geigy water contamination case in Toms River, NJ. Her additional health-related coverage includes death and dying, skin care, and autism spectrum disorder. Learn about our editorial process Share Tweet Email Print Alex Dos Diaz / Verywell 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: 1-2Ebola: 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. 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. What Do We Know About the Omicron Variant So Far? Was this page helpful? Thanks for your feedback! Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life. Sign Up You're in! Thank you, {{form.email}}, for signing up. There was an error. Please try again. What are your concerns? Other Inaccurate Hard to Understand Submit 0 Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to 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.