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More Evidence Of The Safety And Effectiveness Of Vaccines

One of the basic human “needs” is the desire for simplicity. We have limited cognitive resources, and when we feel overwhelmed by complexity one adaptive strategy is to simplify things in our mind. This can be useful as long as we know we are oversimplifying. Problems arise when we mistake our schematic version with reality.

In this same vein we also like our narratives to be morally simple, so there is a tendency to replace the complex shades of gray with black and white. This is perhaps related to cognitive dissonance theory. We have a hard time reconciling how someone can be both good and bad, or how a good person can do bad things. So there is also a tendency to see people as all good or all bad. We can transcend these tendencies with maturity and wisdom, but that takes work.

A good example of the desire for simple moral clarity is the anti-vaccine movement. Their world is comprised of white hats and black hats (guess which one they perceive themselves as wearing), as evidenced by the blog posts and comments over at Age of Autism. There is a certain demand for purity of thought and message that seems to be getting worse over time in a self-reinforcing subculture. Many now see their struggle in apocalyptic terms.

The desire for simplicity even extends to factual claims. They oppose vaccination, and so they tend to make every argument against vaccines possible – even arguing, against all the evidence, that vaccines do not work. If vaccines were effective but carried significant risks, that would cause a genuine dilemma (cognitive dissonance). But if vaccines are both ineffective and risky, there is no dilemma, the cognitive dissonance is resolved, and the brain is given a nice dose of dopamine as a reward.

This means that defenders of science-based medicine have to counter anti-vaccine propaganda stating that vaccines do not work. For example, the data on measles is overwhelmingly clear, but this has not stopped vaccine deniers from distorting the data to argue that measles just happened to decline all on its own. It’s a massive exercise in not seeing the forest for the trees. Deniers look for anomalies in the data (usually artifacts of data collection) and then use that to call the big picture into question. Or they confuse death rates with incidence rates (death rates can decline just by improvements in medical care – this does not mean that the spread of the disease was decreasing). Meanwhile the big picture is dramatically clear – vaccine introduction lines up nicely with plummeting disease incidence.

So forgive us if we take the time to point out when further evidence comes to light that vaccines are effective public health measures. A recent study published in Pediatrics reviews the evidence for the effect of the varicella (chicken pox) vaccine on varicella-related deaths. They found:

During the 12 years of the mostly 1-dose US varicella vaccination program, the annual average mortality rate for varicella listed as the underlying cause declined 88%, from 0.41 per million population in 1990–1994 to 0.05 per million population in 2005–2007. The decline occurred in all age groups, and there was an extremely high reduction among children and adolescents younger than 20 years (97%) and among subjects younger than 50 years overall (96%). In the last 6 years analyzed (2002–2007), a total of 3 deaths per age range were reported among children aged 1 to 4 and 5 to 9 years, compared with an annual average of 13 and 16 deaths, respectively, during the prevaccine years.

That’s an impressive decline, if the absolute numbers are low. But when you are talking about childhood deaths, any reduction is welcome. Although it was not covered in this study, other studies also have looked at varicella incidence and hospitalizations, also finding a dramatic decrease. For example:

The vaccination program reduced disease incidence by 57% to 90%, hospitalizations by 75% to 88%, deaths by >74%, and direct inpatient and outpatient medical expenditures by 74%.

All of this data is also with the single doses vaccine, which was found not to produce adequate antibody levels in some children. The current recommendation is for a second follow up dose to boost immunity levels. It is probable that the two-dose vaccine will produce even more impressive results.

And so as not to oversimplify the picture – the varicella vaccine did come with a possible unintended consequence. Previous generations were often exposed to chicken pox in children throughout their life, resulting in a natural immunity booster. With the near elimination of chicken pox due to the vaccine, older adults have waning immunity and this has possibly led to an increase in herpes zoster. Once infected with varicella the virus is never completely eliminated from the body. It goes dormant in the dorsal root ganglia (packets of sensory nerve cells just outside the spinal cord) and can be reactivated later in life. It’s possible that decreasing antibody levels in older adults who are no longer getting exposed to cases in children are allowing more cases of zoster to occur.

The data on this is currently mixed. Models predict an increase, but actual surveillance has produced unclear results. The worst case scenario is that the older generation will experience an increase in herpes zoster, but this will be a temporary effect as the next generation will never have had chicken pox due to the vaccine. There is also available a varicella zoster vaccine to reduce the risk of zoster in the at risk generation.


There is a large and growing body of scientific data from which we can draw a few very reliable conclusions. Vaccines work. The general concept is sound, and specific vaccines have clearly been effective in significantly reducing (and in two cases eliminating) infectious disease. They are not risk free, but the incidence of adverse events is orders of magnitude lower than the benefits of the available vaccines.

We need to continue active surveillance of vaccine safety and effectiveness, as well as tight regulation of vaccine manufacturing. Vaccines are an important public health intervention, and we need to watch the vaccine program closely.

Despite this, vaccine opponents have continued to argue that vaccines are not safe or effective. Thankfully the data is so clear that the public is largely ignoring them.

*This blog post was originally published at Science-Based Medicine*

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