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.

vaccination coverage

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.


12 thoughts on “Vaccination Coverage and Herd Immunity

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  2. There are lots of great explanations of population- or herd-immunity. One of the best, wrt whooping cough in particular, is Adam Finn’s video and its graphics

    Adam’s video also explains how whooping cough vaccines (like the disease itself, in fact) don’t produce long-term immunity. The disease is most dangerous in babies; so stopping babies getting the disease is our main priority. But because we don’t get long-term immunity, the disease always circulates in the community (albeit in older age groups); so it remains particularly important to protect babies. Adam’s video explains how and why the programme to vaccinate pregnant women has been so necessary and so effective in protecting babies.

  3. The evidence for “coccooning” is mixed. It’s a good idea in theory; but very hard to implement in practice.

    The UK approach is more robust. When you vaccinate a pregnant woman – especially if she’s been “primed” by previous immunisation – her antibody levels rise dramatically. During the last few weeks of pregnancy antibodies cross the placenta from mother to baby. If the mother has high levels of anti-whooping cough antibodies at that time, the antibodies protect the baby until it can be vaccinated and start producing its own antibodies.

  4. My previous attempt to explain herd immunity (requires free registration):

    English PM. Herd immunity and reproduction numbers. Vaccines in Practice 2009;2(2):7-9. (

    A couple of other model/demonstrations:

    And more links available via

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