One of the topics discussed in "The Vaccine War" was herd immunity or community immunity. The concept behind herd immunity, is that if a large enough proportion of the population gets immunized against a particular disease, more vulnerable populations who are unable to be immunized will also receive some protection from the disease. Vulnerable populations who would need protection via herd immunity include populations such as pregnant women, young infants and elderly people who are unable to receive vaccinations themselves. The particular proportion of individuals that must be immunized in order for herd immunity to work varies for different diseases with small pox requiring 83-85% of the population to be immunized and Diphtheria requiring 85%. While these may seem like relatively large proportions of the population, vaccination rates for small pox reached high enough levels to effectively eradicate the disease and render the vaccine unnecessary for future populations. By choosing to not vaccinate, not only are vulnerable populations put at risk, the length of time that vaccinations for certain diseases must be around is increased. The choice to not vaccinate perpetuates a cycle of increasing numbers of vaccinations as the threshold is never reached beyond which vaccinations for certain diseases are no longer necessary.
References:
http://www.niaid.nih.gov/topics/pages/communityimmunity.aspx
http://en.wikipedia.org/wiki/Herd_immunity
Coleen,
ReplyDeleteI loved your post. As I did research into the Rotavirus I was more and more convinced that the argument against vaccinating in the United States, for the Rotavirus specifically, was correct. It wasn't until I hearkened back to "The Vaccine War" and remembered the argument for herd immunity that I could successfully defeat the growing argument in my head. I wonder if a stronger case was made by pediatricians for herd immunity instead of the statistics, like the ones I posted in my blog post, if more parents would be apt to vaccinate when they are vacillating on the side of not vaccinating. Most parents do vaccinate their children, I know that I have, but these small communities which don't are definitely of concern for the at risk groups you mention. Great post.
I appreciate this post Colleen. It would be interesting to see how these statistics are calculated. It definitely seems to be related to how contagious the said disease is. It would also be interesting to see if this is related to the calculations the CDC does to phase out vaccines when a disease, like small pox, is officially "extinct".
ReplyDeleteGood question. Wikipedia has a nice article on modeling infectious disease: http://en.wikipedia.org/wiki/Mathematical_modelling_of_infectious_disease
ReplyDeleteScroll down to the section on "Mathematics of mass vaccination."
The ratios discussed in the Wikipedia article translate to the percentages that Colleen wrote about. Getting estimates for the variables in the mathematical model is done through statistics.