Last fall I wrote about how contagious Ebola is compared to other diseases, using a mathematical calculation called R₀ (R naught).  R₀  is a tool we use in epidemiology to estimate the average number of people one sick person will infect.  The R₀  for measles is off the charts.  It has one of the highest R₀ (from 12-18 compared to an R₀ of 1.5 – 2 for Ebola).  In fact, 90% of unvaccinated (non-immune) persons exposed to a person with measles will come down with measles.

The R₀ helps estimate the vaccine coverage needed to stop the spread of measles.  For R₀  = 18, a minimum of about 94% of the people in a community need to be vaccinated to reach herd immunity.  The goal of public health in a measles outbreak is to bring the R < 1.  When that happens, spread of a disease stops and sick patients don’t infect others.  In the case of measles, the best tool is increasing herd immunity and keeping the sick people from exposing others while contagious.

Anybody that wants to pitch in to stop Arizona’s measles outbreak can take one simple action.  Make sure you and your family are vaccinated.