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Understanding How Contagious Covid-19 Is

By Meghan Giera, MPH Intern

Contact Rate Chart

What is R0?

R0, pronounced “R naught,” is a number used by scientists to indicate how contagious an infectious disease is. When one person is infected, the R, or reproduction number, tells us the average number of people who will become infected from contact with that original infected person.

  • An R0 less than 1 indicates each infected person cause less than one infected person. The disease spread will slow and eventually stop.

  • An R0 equal to 1 means each infection will cause one new infection. The disease is stable but will not cause an outbreak.

  • An R0 greater than 1 means each infection will cause more than one new infection. The disease will spread more quickly and may lead to an outbreak.

    How many will each sick person infect?

  • The 2009 influenza A (H1N1) pandemic had a median, or average value of R0 = 1.47

  • The Seasonal influenza has a median number of R0 = 1.27

  • New research shows that Covid-19 has a median R0 = 5.7

    Why this is important

    The R0 is used to calculate an entire population’s susceptibility to an infection. Aggressive countermeasures, such as social distancing, are selected for our states and cities based on the calculated numbers of infected. With an R0 of 5.7, Covid-19 is easy to catch and spread. A contagious person who comes into contact with many other people will cause Covid-19 to spread more quickly. A high contact rate will lead to a higher R0 number. If the infected person remains at home, limits his or her contact with others, and continues to social distance, the disease will spread more slowly.

    Spread with a High Contact Rate

    Since there are many mild cases and asymptomatic infections, it is important for everyone to limit their contact with others. A lower contact rate will help slow the spread and decrease the rate of transmission. Please visit the CDC’s page on Social Distancing, Quarantine, and Isolation for more information about how you can slow the contagion.

    Content Sources

    Biggerstaff, M., Cauchemez, S., Reed, C. et al. Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature. BMC Infect Dis 14, 480 (2014). Sanche S, Lin YT, Xu C, Romero-Severson E, Hengartner N, Ke R. High contagiousness and rapid spread of severe acute respiratory syndrome coronavirus 2. Emerg Infect Dis. 2020 Jul [date cited].