Vaccines Cause Optimism

A question posed by a parent in an antivaccine Facebook group:

Measles was declared “eliminated” in Canada in 1998, and in the US in 2000.  Perhaps this has caused people to become complacent, which has enabled the antivaccine movement to gain a foothold to the extent that several countries in Europe have recently lost their measles-free status (https://www.cnn.com/2019/08/29/health/europe-measles-outbreaks-scli-intl/index.html), and the US is at risk of losing its status in October (https://www.cnn.com/2019/08/28/health/us-measles-elimination-status-in-jeopardy/index.html).

Anti-vaxxers often claim support from peer-reviewed sources – for example, one article in Natural News cited a paper about measles vaccination from the American Journal of Public Health (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1646939/).

Taken out of context, this quote might be seen as critical of the effectiveness of the vaccine:

“Vaccine failures among apparently adequately vaccinated individuals were sources of infection for at least 48 per cent of the cases in the outbreak.”

But reading further, the conclusions are much more positive:

“There was no evidence to suggest that waning immunity was a contributing factor among the vaccine failures. Close contact with cases of measles in the high school, source or provider of vaccine, sharing common activities or classes with cases, and verification of the vaccination history were not significant risk factors in the outbreak. The outbreak subsided spontaneously after four generations of illness in the school and demonstrates that when measles is introduced in a highly vaccinated population, vaccine failures may play some role in transmission but that such transmission is not usually sustained.”

It would seem that much of the success of the anti-vaccine movement can be attributed to the fact that most people do not understand probability and statistics (hence the success of casinos and government lotteries – see also https://www.harpercollins.ca/9781443453097/knock-on-wood/).

For example, let’s take a hypothetical population of 2001 students, of which 2000 are vaccinated and 1 is not, and expose them to measles.   For this case, we’ll assume that the probability of contracting the disease is 1% for a vaccinated person, and 50% for an unvaccinated person (note that measles is actually much more contagious than this conservative estimate).  So, the 1 unvaccinated person would have even odds of escaping the disease, while it is very likely that at least 20 of the 2000 vaccinated people will get sick.  So, it would not be surprising if the unvaccinated person stayed healthy, and only vaccinated people got sick.  But if you had 2000 unvaccinated and 1 vaccinated, the vaccinated person would have a 99% chance of not getting sick, while the expectation would be that about 1000 people would come down with measles, one of whom would likely get encephalitis resulting in permanent brain damage or death (not to mention the risk of a few dozen having other complications such as pneumonia, or permanent deafness and blindness).