Herd immunity is the additional protection against a disease that a community acquires when the vaccination rate against that disease crosses a certain threshold.
Wow. That’s quite a mouthful. Let us give an example and then try to explain the details of the concept.
Let’s say that we immunize 92 percent of the population of a community against polio and that the vaccine is 100 percent effective. We might reasonably expect to see the number of cases of polio decrease by 92 percent, leaving the remaining 8 percent of cases for those people who were not vaccinated. However, in reality, the number of cases of polio decreases by over 99 percent. That extra 7 percent of protection is due to herd immunity.
Herd immunity is a major societal benefit of mass vaccinations. If enough members of a community are vaccinated and thus protected against a disease, then even if the disease reappears, it will only be able to spread to a few people. Because there are no longer very many susceptible hosts, the transmission of the disease will die out quickly. Furthermore, since most illnesses are passed from person to person, one unimmunized person living in a highly immunized community is unlikely to be exposed to any given disease. That unimmunized person is protected by the immunity of the herd of people around him.
For most illnesses, the estimated herd immunity threshold is a vaccination rate of around 90 percent. This means that if 90 percent of the population, or “herd,” is successfully protected against the illness, the disease will die out because there aren’t enough susceptible hosts to perpetuate its transmission.
Note that herd immunity does not require perfection. That’s lucky, because no vaccine is 100 percent effective. In addition, not all children are vaccinated. Some children have medical exemptions against the vaccinations, and some parents refuse vaccinations for their children based on other reasons. However, if the number of vaccinated people is high enough, the disease is unlikely to resurface or cause an epidemic.
Of course, if the protection percentage drops too far below the herd immunity threshold, then the disease has room to spread and grow. Since no vaccine is 100 percent effective, the loss of herd immunity puts even vaccinated people at risk. In 1994 in the former Soviet Union, the collapse of the public health system led to decreased immunization rates and an epidemic of diphtheria. Whereas most developed countries might have less than twenty-five cases a year, the Soviet Union had almost forty thousand cases that year. Given the disease’s 3 percent fatality rate, twelve hundred people died that year who might have lived had their national immunization program been up to date. In addition, over thirty-eight thousand people were sick, and possibly hospitalized, with the disease.