Thread by @BillMiller_Epi

As we think about COVID-19, a basic tweetorial about R0 (basic reproduction (or reproductive) number) and Re (effective reproduction number).

R0 = average # of people who will be infected by an individual in an entirely susceptible population. #EpiTwitter #FlattenTheCurve 1/n

R0 is a function of 3 key factors:
c = Contact rate: how often people are close to other people
p = Probability of transmission given contact with infectious person: how infectious the virus is
D = Duration the virus can be transmitted

R0 = c*p*D
2/n

Re (effective reproduction #) adds in the proportion of people who are susceptible (s):

Re = R0 * s = c*p*D*s

If everyone in the population is susceptible, s=1, & Re=R0
3/n

R0 is not a constant. Each factor has a distribution. p will depend on the # of organisms a person is exposed too. Generally, if someone is sicker, they will shed more viruses. And with more viruses shed, transmission is more likely. 4/n

c, contact rate, varies from one setting to the next. This parameter is the basis for social distancing. If we can drive c close to 0, then R0 (or Re) will be driven lower. The more we stay away from one another, the more we protect each other. 5/n

s, the proportion susceptible, is critical for the concept of herd immunity. SARS-CoV-2, the virus that causes COVID-19, seems to induce short-term immunity but we don’t know for sure if that immunity will be long-term. 6/n

Fundamentally, the one thing that we humans have control over is c, the contact rate. We all control that to some extent. Do your part and keep your distance (6 feet!). Wash your hands. Disinfect your surfaces. Stay healthy. Protect yourself. Protect others. 7/7