Vaccines and Herd Immunity: Why Lower Vaccination Rates are Potentially Dangerous

As the anti-vaccine movement has progressed in recent years, many people on both sides of the issue have found themselves wondering whether or not lower rates of childhood vaccination actually pose a threat to the broader population. Intuitively, many reach the conclusion that there is no potential for harm to any but those who have not received vaccinations because all other individuals are still protected. However, there is another factor that comes into play, which is the concept of herd immunity. Unbeknownst to many, lower rates of childhood vaccination have the potential to compromise herd immunity and set the stage for epidemics of diseases that have been under control in most countries for decades.

How is Herd Immunity Achieved?

Although it is a medical field, epidemiology (the study of the spread of infectious diseases) relies heavily on mathematics to model how diseases spread within populations. One of the fundamental mathematical elements of this field is the basic reproductive number, which is effectively a measure of how many new cases any given case will result in. Diseases with a basic reproductive number of less than 1 do not have the capacity to become epidemics, as they will extinguish themselves in any given population naturally.

The measure of the rate of vaccination needed to achieve herd immunity is derived from the basic reproductive number of the given disease. To determine this needed vaccination rate, the inverse of the basic reproductive number is subtracted from 1. The result is then expressed as a percentage. For example, mumps, which has a basic reproductive number of 4 in some populations, requires a 75 percent vaccination rate within those populations to achieve herd immunity. It should be noted that the basic reproductive number in any population will vary based on a number of different factors, including acquired immunity from previous cases of infection.

How Lower Vaccination Rates Endanger Entire Populations

Given this information, it can clearly be seen that herd immunity is dependent on vaccination rates being as high as possible. However, the argument still holds that those who have been vaccinated have a degree of individual protection, regardless of herd immunity status. While there is some truth to this, the reality is far more complex. To begin with, a population will almost always include some individuals with compromised immune systems who are particularly susceptible to infection. For these individuals, herd immunity within a population offers a measure of protection, as epidemics of diseases against which the population as a whole has herd immunity are extremely rare. Even vaccinated individuals are put at risk when herd immunity is compromised because of the ability of many viruses to mutate as they spread. If a virus changes enough through multiple transmissions, it can infect even someone who has received effective vaccination against the original virus.

In order to fully understand the scope of the debate over vaccines, these facts are essential. Many who oppose vaccination believe that lower rates of childhood immunization present no credible risk to the general population. As with many things, however, the truth in the matter is more nuanced. Low rates of vaccination endanger everyone within a population, not just those who do not receive vaccines.

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