Below is a short, disorganized note I wrote to a friend who wasn't a fan of childhood vaccinations. I thought I would put this out there for critique and hopefully a productive public discussion:

---

Before I begin, please note that unfortunately access to most online medical journals isn't free. Please let me know if you'd like a copy of any of the papers I've cited.

When reading the latest news headlines espousing either pro- or anti-vaccination studies, it's important to look behind the news article to the study itself. In the medical field, we call this "evidence-based medicine" - research is evaluated as it is reported, and how we practice medicine changes based on how strong the evidence is for or against a particular practice. A small study including patient interviews is unlikely to change practice widely, whereas a large randomized and controlled trial including 10,000 patients is likely to change our practice as a profession. One fairly recent example of this is the Cochrane review on oral anticoagulants (i.e. Warfarin) versus anti-platelet drugs (such as Aspirin) in those with Atrial Fibrillation (i.e. an irregular heartbeat) for the prevention of the stroke. The idea is that an irregular heartbeat can cause blood clots to form in the heart, that then "break off" and travel to the brain, causing a stroke. The Cochrane Review, which is essential a study-of-studies on a particular subject, showed that drugs such as Warfarin are about 1/3 more effective than Aspirin in preventing stroke. This study included 8 randomized trials, and over 9000 patients, and although most physicians were already giving Warfarin to patients with an irregular heartbeat, this study confirmed that this was indeed the "best medicine" for the patient. For more on evidence-based medicine, this link covers the different "levels" of evidence that are important when discussing vaccination research.

Because the MMR (Measles, Mumps and Rubella) vaccine seems to be the primary target for anti-vaccine groups and the main concern for worried parents, I'll discuss the evidence for the MMR vaccine briefly. We're fortunate that in 2008 the Cochrane Library published "Level 1++" evidence on the topic - a meta-analysis of all published trials regarding the MMR vaccine.

Their published objective:

1. To review the existing evidence on the absolute effectiveness of MMR vaccine in children (by the effect of the vaccine on the incidence of clinical cases of measles, mumps and rubella).
2. To assess in children the worldwide occurrence of adverse events, including those that are common, rare, short and long-term, following exposure to MMR.

This "study-of-studies" started very broad in trying to identify adverse outcomes of vaccination - they started with 5000 articles. They narrowed this down to 139 articles on the subject since 1966, and after selecting only retrospective and prospective studies on the subject they had 31 published articles to review.

The results of this study showed that there are adverse (bad) effects of the vaccine, but that they are very rare and generally not serious. They were unable to show that the vaccine was effective as a whole (because there is no specific research on this topic to date), but noted that other studies have show that each component of the vaccine is effective individually. For example, there is Level 2+ (a.k.a. "good, but not great") evidence showing that the Mumps component in the MMR vaccine is at least 88% effective when both required shots are given. The effectiveness rate for Measles in the MMR vaccine is similar.

According to the CDC, almost 3 in every 1000 people infected with Measles will die - stated another way, the mortality rate from Measles infection is about 0.3%. In the current vaccinated Canada, incidence of Measles is in the tens per year, which means that the chance of dying from Measles is 3/1000 x 20/35,000,000, or essentially negligible. Rates for other vaccine-preventable diseases are roughly equivalent (pertussis ("whooping cough") is quite a bit more infectious), so this begs the question of "why vaccinate?", if there is even a slight chance of adverse effects.

This is certainly a reasonable question, that I would counter with three points:

1. The only reason that your children have a negligible chance of contracting these life-threatening infections is because most parents in Canada have chosen to vaccinate their children, reducing the incidence of infection significantly. With Measles, for example, rates have gone from greater than 90% to virtually nil. Pre-vaccination, your children would have had a 30,000-fold increased chance of dying from Measles than in a modern day plane crash - post-vaccination, all we have to worry about is the plane crash.

2. There is a small proportion of children who cannot receive vaccinations because their immune system is compromised (for example, in early childhood cancers, genetic defects, or other serious illness), and rely on herd immunity to protect them from these deadly diseases. By not vaccinating children that are able to handle the vaccines and become immune, we put these sick children at risk.

3. Although definitive evidence of the risks of adverse effects has yet to be carried out for most childhood vaccines (I hope it does eventually!), the dearth of credible case studies demonstrating serious adverse effects over the 200+ years of routine vaccination is strong evidence in itself of their safety. Remember that still, to this day, we have relatively poor evidence for the causative link between smoking and lung cancer - what has changed the consensus opinion is the long time period and the very strong correlation between the two. Conversely, I am unaware (and certainly haven't heard differently in medical school) of any correlation, strong or otherwise, between childhood vaccinations and an increased risk of adverse effects. As you would know, the single "peer reviewed" study done on the link between autism and the MMR vaccine has been fully retracted by The Lancet and 10 of the 12 original authors of the paper.

One last comment: while researching this further I have found a rather disturbing trend in anti-vaccination groups to lend an undue weight to case reports and anecdotal evidence supporting the link between vaccinations and adverse effects. Remember that there are "levels" of evidence, with case reports and other anecdotal evidence making up the bottom, or least reliable, level. Just as I (and the Cochrane authors) have discounted case reports in demonstrating the safety and efficacy of the MMR vaccine, so too does the anti-vaccine movement need to rely on unbiased and controlled trials. To date I have yet to see any of the studies cited by the Vaccination Risk Awareness Network that meet this criteria.

Concerns about immunization safety often follow a pattern. First, some investigators suggest that a medical condition of increasing prevalence or unknown cause is an adverse effect of vaccination. The initial study, and subsequent studies by the same group, have inadequate methodology, typically a poorly controlled or uncontrolled case series. A premature announcement is made of the alleged adverse effect, resonating with individuals suffering the condition, and underestimating the potential harm to those whom the vaccine could protect. The initial study is not reproduced by other groups. Finally, it takes several years to regain public confidence in the vaccine. Adverse effects ascribed to vaccines typically have an unknown origin, an increasing incidence, some biological plausibility, occurrences close to the time of vaccination, and dreaded outcomes.

— From Wikipedia: Vaccine Controversies