If you didn’t get a flu shot, you are endangering more than just your own health. Calculations of herd immunity against common diseases don’t make exceptions.
The necessary level of immunity in the population isn’t the same for every disease. … The number of people infected by each contagious individual is known as the “basic reproduction number” of a particular microbe (abbreviated R0), and it varies widely among germs. … Note that the R0 isn’t related to how severe an infection is, but to how efficiently it spreads.
Determining the R0 of a particular microbe is a matter of more than academic interest. If you know how many secondary cases to expect from each infected person, you can figure out the level of herd immunity needed in the population to keep the microbe from spreading. This is calculated by taking the reciprocal of R0 and subtracting it from 1. For measles, with an R0 of 12 to 18, you need somewhere between 92 percent (1 – 1/12) and 95 percent (1 – 1/18) of the population to have effective immunity to keep the virus from spreading. For flu, it’s much lower — only around 50 percent.