Wednesday, July 21, 2010

Vaccination: A Love Story

(Dear reader, I apologize in advance for the rather cliche topic of this post...I'm just fed up with all of my comments getting moderated off of age of autism.)

Perhaps one of the most pernicious aspects of the culture war is the anti-vaccination movement. Driven by charismatic celebrities such as Bill Maher and Jenny McCarthy, an army of soccer moms, and a handful of ethically dubious fringe scientists, the "anti-vaxers" represent one of the greatest extant immediate threats to general public health in the developed world. Since its modern beginnings in the early 1990's to the present day, the movement has accrued a cult-following of otherwise well-meaning people that has done nothing but hinder the progress of science and cause unnecessary human fatality. Contributing to the movement is both a general lack of understanding of the science behind vaccines among the general public and clinicians, and a not-wholly-irrational public distrust of the pharmaceutical industry. Addressing this issue will require effective and persuasive communication of vaccine science to both the general public and practicing clinicians.

As a safe and effective preventative counter-measure to infectious disease, vaccination is far from a modern concept. Primitive vaccination protocols can be found in ancient aryuvedic texts from India, and the procedure was widely practiced in the Muslim world until it was introduced to western medicine by Lady Mary Montague and (more famously) Edward Jenner in the 18th century. While the technology of vaccine production and delivery has changed over time, the basic concept that a specific immune response can be "primed" in a person to render immunity to a pathogen prior to exposure remains. In order to properly explain the mechanism at play here, it is first necessary to briefly explain how immunity works.

In the most reductive sense, the human immune system differentiates between things that are supposed to be in the body (e.g. one's own serum proteins and cells) and things that are not (e.g. viral coat proteins, bacteria, and even cancer). To accomplish this task, and eliminate those items that fall into the latter category, human immunity functions through two broad mechanisms. Innate immunity is just that, innate, upon contact with any foreign biotic material, it functions to quickly and efficiently eliminate it. Sometimes, however, this non-specific first line of defense is breached and, in the case of infectious disease, the body needs to break out the heavy artillery.

The components of adaptive immunity represent the body's version of guided missiles and surgical strike commandos. Upon exposure to an antigen, macrophages (the "storm troopers" of innate immunity) present the antigen to adaptive immune cells and trigger proliferation of cells specific to that antigen. These adaptive immune cells fall into two categories, B-cells and T-cells. B-cells produce antibodies, proteins capable of recognizing a specific antigen and facilitating its destruction; once the body starts producing antibodies against a specific antigen, it will continue to do so--granting immunity--throughout life. T-cells are a bit more diverse, and fall into several subcategories, but their general role is to recognize and kill infected and pathogenic cells. Following exposure to a specific antigen, "memory T-cells" specific to that antigen persist throughout life and can proliferate quickly to mount a specific response in the event of re-exposure. Vaccines induce adaptive immunity to a particular pathogen so that, upon exposure, the body already has a specific response.

Modern vaccines fall into five broad categories, attenuated, killed, subunit, toxoid, and conjugate.

Killed and attenuated vaccines are technically the simplest to produce and the earliest vaccines generally all fall under these two categories. These vaccines work by introducing killed, noninfectious, or weakly infectious versions of the infectious pathogen into the body thereby simulating infection and inducing an adaptive immune response. While certain attenuated vaccines may cause infection in immunocompromised individuals, and many of these vaccines must be refrigerated to be kept "fresh", they have become one of the safest and most reliable counter-measures to infectious disease in use today.

Subunit, toxoid, and conjugate vaccines work by introducing biomolecules specific to a given pathogen rather than the pathogen itself. Vaccines in this category can be used to develop an adaptive immune response against bacterial lipopolysaccharides and even small organic molecules. Because, with the exception of some toxoid vaccines, recombinant DNA technology is required to produce them, vaccines in these three categories did not enter wide-spread clinical use until the latter half of the 20th century. Because they contain no potentially infectious particles, these vaccines pose no risk of causing infection. Additionally, some of them can even be stored "dry", greatly increasing their shelf-life. These vaccines are highly safe, with the clinical trials for a few of them producing no statistically significant life-threatening effects.

Re-enter the anti-vaxers. The most common ailment blamed on vaccinations is autism. Much of the momentum for this belief was provided by a now-retracted-and-thoroughly-disproven paper published in the Lancet in 1998. No peer-reviewed publications have ever presented even a hypothetical pathogenic mechanism for such a causal linkage; in fact, much of the anti-vaccination fervor has focused on pseudoscientific vagueries such as "toxic load" and "vaccine load."

"Toxic load" as defined by the anti-vaxers refers to the idea that trace chemicals or preservatives present in the vaccine induce autism. Several studies have thoroughly discounted this concept. The only potentially scientifically valid tenet underlying this idea is several public health studies that have shown a (barely) statistically significant correlation between environmental mercury content (defined variously as soil concentration, drinking water concentration and atmospheric concentration) and local prevalence of autism. Unfortunately, a biologically insignificant amount of mercury given as a bolus (as in a vaccine) and environmental exposure throughout development are still two very different things, and treating autism as heavy metal poisoning can have lethal consequences.

"Vaccine load" is the idea that the normal vaccine schedule gives too many vaccines at once, thereby overwhelming a child's immune system. This concept blatantly disregards that the human body is constantly under attack from pathogens and has evolved to survive under constant attack from pathogens--the human body is more than capable of dealing even with a relatively high titre of thousands of antigens at once. Not only is the normal vaccine schedule safe, adhering to it is necessary for the maintenance of "herd immunity" and general public health.

Vaccination is one of the oldest medical procedures still in use today, and continues to be one of the most effective tools available against infectious disease. Outlining the flaws in the anti-vaccination movement would produce a blog entry far longer than I am willing to write, but there will certainly be more to follow on this topic.

1 comment:

  1. A concise editorial giving a bit of history about how this misconception came about:

    Good empirical evidence disabusing the notion that vaccines cause autism: (a review article)