Vaccination Coverage and Herd Immunity

I’ve talked about vaccination and herd immunity on this blog before, but I think it’s important for me to emphasize how INCREDIBLY IMPORTANT it is to get vaccinated.  The importance of vaccinating most of the population is usually explained using mathematics, because scientists study the spread of pathogens by using mathematics.  But today, I’m going to try to explain it with cartoons and pictures, instead of math.

Without explaining the math, I’ll say that there are some “magic numbers” for vaccination.  These numbers are unique to each pathogen/disease.  For instance, for whooping cough, a disease that can make make babies very sick, the “magic number” is between 92 and 94.  That is, 92-94% of people must be vaccinated in order to prevent disease epidemics of whooping cough.  If that magic number – called the herd immunity threshold – is reached, babies are indirectly protected from whooping cough.  If not, you can expect outbreaks of whooping cough.

So, you might be wondering if there will be outbreaks of whooping cough where you live.  Check out this graphic that was published in Scientific American last year.  If your state’s bar is red – that is, if you live anywhere except Nebraska – you can expect epidemics of whooping cough in your state in the near future.  And while it looks like nobody will be seeing Mumps epidemics any time soon, you can expect to see Measles epidemics in many states.

At one point, we’d nearly eliminated whooping cough in the United States by vaccinating children with the DTP vaccine.  Here’s a graph from the CDC showing that after we started using the DTP vaccine around 1950, whooping cough (also called pertussis) almost completely disappeared.  But in the past decade or so, the number of cases reported each year has been increasing. That is likely due to a decline in the effectiveness of the newer pertussis vaccine, rather a decline in vaccination coverage.