Vaccinations and “herd immunity”

Cooperating to save lives

One of the more interesting phenomena in the realm of epidemiology is how infectious disease affects large populations. Though the patterns of morbidity and mortality will fluctuate, taken as a whole they tend to adversely impact certain subpopulations more than others. For example, the very young and very old can have higher rates of illness and death from many infectious disease.

The obvious benefit of vaccinating against certain diseases in all types of animals has been nothing less than spectacular and revolutionary. From human Small Pox to Canine Rabies, the resulting eradication or reduction of disease exemplifies the power it has had to positively impact health- especially in many of the most susceptible groups. In addition to the obvious effects of protecting an individual from a disease, vaccinating whole populations has another powerful effect.

By preventing the spread of infection, vaccination can reduce and sometimes eliminate the risk of disease transmission in general -otherwise known as herd immunity. Dr Panagiotopolulos of the Institute of Child Health in Athens (Egypt) defines herd immunity as “The indirect protection from infection of susceptible members of a population, and the protection as a whole, which is brought about by the presence of immune individuals.” This means that, for example, in addition to having the ability to protect individuals from a certain disease, vaccinating a population incurs another level of protection -like an umbrella- and blunts the propagation of the disease. Many people are vaguely aware of this property, but don’t realize just how crucial it is in any cases for providing optimal protection.

This phenomenon can be described mathematically and provides the theoretical foundation for many of the herd immunity effects observed in real vaccination programs. An infection will usually spread via the “mass action principle” and is a function of the number of susceptible individuals. The equation is described as follows:

C_t+1/C_t = f (S_t), where C is the number of infected cases, S the number of susceptibles, t a given time period, t+1 the next time period. (This can also be expressed as: C_t+1 = S_t C_t r, where r is a transmission parameter¹).

What is so interesting about this equation is that it can help describe the dynamics of how infections spread throughout populations. Dr. Panagiotopulos notes “ It was introduced in the 1900’s and helped understand the dynamics of epidemics of diseases like measles: as the infection spreads during an epidemic, the number of infected cases in each successive time period initially increases while the number of susceptibles in the population decreases; therefore, there will be a point when susceptibles become sparse and the number of new cases in each successive time period decreases; and, finally, susceptibles are so scarce that there is no more than one new case for each case in the previous time period, and the epidemic fades out although a number of susceptibles have not been infected.”

Several potential effects can be gleaned from the movement of diseases through these populations and are usually related to the beneficial effects of vaccination. In general, the propagation of diseases are either blunted or even stopped, especially if a large part of the population is protected (herd immunity). However, there can be adverse effects (“perverse effects”) in some cases and these are dependent on the specific disease in question and the level of protected vaccinates.

For example Rinderpest in Africian cattle, Chicken Pox and Rubella in humans have peculiar characteristics that require special consideration. Rinderpest vaccination can be more targeted to focal outbreaks due to a synergistic herd immunity that makes immunizing the hold population less beneficial. Rubella, if vaccination campaigns are less than ideal, has the unfortunate characteristic of protecting one group while actually placing another at higher risk than before.

In these cases, it is vital that other factors are taken into account before implementing broader vaccination campaigns. Of course, this is a point that anti-vaccination advocates get spectacularly wrong as they misrepresent this particular form of herd immunity (i.e.; claiming that all vaccines do this, or taking it out of context) and use this false knowledge to decry vaccinations in general.

One of the most important points to consider is that vaccination protocols need to cover a high percentage of populations to reduce complications in disease dynamics –especially in these special instances- to optimize the herd effect. In other words, a significant number of unvaccinates can put a larger population at risk². In the cases where it really matters, the selfishness of anti-vaccine advocates can directly cause harm to others.

The bottom line is that herd immunity is an extremely potent natural phenomenon that provides effective protection –sometimes even the eradication- of many diseases. The implementation of vaccination campaigns need to be realized with care and preparations made so that the threshold of proper herd immunization levels can be reached. Vaccines protocols can be modified as other factors might present themselves (synergistic immunity as in the Rinderpest case).

With people, the best results seem to come with cooperative efforts (there may be -though a very touchy subject- a place for obligatory vaccines in some cases). Depending on the country, different educational and culturally sensitive strategies may improve the initial conditions for starting vaccination programs. Perhaps similar strategies may appease some in the anti-vaccine crowd in the US, though many in this group seem heavily biased against science and reason- at least when it comes to vaccines.

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(1) Dr. Panagiotopulos adds "The latter expression explains the name of the "mass action principle", which was given by analogy to the "law of mass action" in chemistry, according to which the velocity of a chemical reaction is a function of the concentrations of the initial reagents."

(2) Those small number of individuals who can not be vaccinated for whatever reason (i.e.; illness, immune compromise) would be theoretically protected indirectly through the herd effect but this is not meant for other wise healthy groups that refuse vaccinations (i.e.; for cultural or belief related reasons) and this can be a big obstacle.