Smallpox was a scourge of humanity for centuries, but in 1977 became the first human disease eliminated by a worldwide vaccination campaign. In Chapter 6 of Tools for Critical Thinking in Biology, I explain how the reproductive rates of disease organisms influence the potential success of vaccination campaigns. Smallpox virus has a much lower reproductive rate than measles virus, making it easier to protect an entire population against the spread of smallpox than measles. This is important because no campaign, no matter how intense, can vaccinate everyone – even if it would be logistically possible to do so, some people can’t be vaccinated because they are too young, too old, or have compromised immune systems.
After the success with smallpox, public health agencies began a campaign to eradicate polio. This has reduced cases of paralysis caused by the polio virus from about 350,000 in 1988 to fewer than 2000 per year since 2001, and only 359 in 2014. These few cases occurred in Nigeria, Afghanistan, and Pakistan; polio has persisted in these countries in part because extremist groups such as Boko Haram in Nigeria have blocked vaccination efforts, sometimes by killing vaccinators.
Public health groups working in Africa have stepped up their efforts and adjusted some of their tactics to gain more support for vaccination from local populations. On August 11, 2015, the Global Polio Eradication Initiative reported that for the first time ever, Africa had gone a full year without a case of polio. The last reported case was in August 2014 in Somalia; if there are no more cases in the next two years, the World Health Organization will declare Africa free of polio, putting us even closer to eliminating a second human disease from the world.
I discuss smallpox and polio briefly in Chapter 6 of Tools for Critical Thinking in Biology, but the main purpose of this chapter is to explain how biologists use models to help answer important practical questions. I describe a simple model that we use to estimate the fraction of a population that must be vaccinated to prevent a disease from spreading and apply this model to measles and whooping cough, two diseases with much higher reproductive rates than smallpox and polio. It will be extremely difficult if not impossible to eradicate these diseases. In fact, there are still large numbers of cases in less developed countries as well as outbreaks in more developed countries with good health care systems. In addition to describing this model in the book, I discuss its ethical implications. Do parents have a social obligation to have their children vaccinated, in order to protect not only their own children but the community as a whole, including those who can’t be vaccinated because of age or compromised immune systems? This bears on a movement to reject routine vaccination that has many adherents in some parts of the US and other countries. For whooping cough, those who reject vaccination make a logical error in thinking about causation. Anti-vaccinators want to attribute recent outbreaks of whooping cough to use of a less effective (but safer) vaccine rather than a decline in the rate of vaccination associated with their campaign against vaccination. In making this claim, anti-vaccinators fail to appreciate that events can have multiple, interacting causes. Decreased rate of vaccination probably acted synergistically with use of a less effective vaccine to cause recent outbreaks of whooping cough. See “Complexities of causation” for further explanation and Tools for Critical Thinking in Biology for other complexities of causation.