PrintCoronavirus Calculations & Infographic
The basic reproduction number (R0) is the expected number of cases directly generated by one case. Based on some of the current data, we made the following assumptions:

  1. Under normal conditions R0=2.5 (each infected person is expected to infect 2.5 other people).
  2. Infected people can transmit the disease for a five-day period while they are asymptomatic.
  3. After five days a person will begin experiencing symptoms, quarantine and no longer infect others.
  4. There is a direct linear correlation between social interaction and R0. Therefore, we reduced R0 by 50% (R0=1.25) or 75% (R0=0.625) and repeated the calculation.

With these assumptions in mind, starting with one infected person, we calculated the total number of people that would become infected after 30 days as
1 + R01 + R02 + R03 + R04 + R05 + R06.  This can also be shown as the following equation:Equation_Post

One caveat is that when R0<1, as with the 75% reduction, the number of cases will actually decline over time and eventually go to zero. This is because each infected person cannot infect 0.625 people, it is either zero, one or more. When it is zero, that transmission chain ends. Exact calculation of the 75% reduction thus requires more complex probability calculations, but we kept things simple for the purpose of illustrating the enormous impact social distancing can have in safeguarding our communities around the world.

Since R0 and time of transmission are still being determined, these numbers should only be considered estimates.

By: Robert A.J. Signer, Ph.D.
Assistant Professor of Medicine
University of California San Diego

Infographic illustrated by:
Gary Warshaw
Art Director

Media Requests:
Lisa Warshaw


Page published: March 20, 2020