..The influenza vaccine presents a useful illustration of how vaccination, rather than strengthening the immune system, can weaken it and cause the host to become more vulnerable to pathogens.
An April 2010 study published in PLoS Medicine found that in Canada receipt of the 2008-2009 seasonal influenza vaccine was associated with an increased risk of illness due to the 2009 H1N1 "swine flu" virus.
The authors of the study hypothesized that this was due to the way the vaccine confers immunity as compared to immunity acquired through natural infection. They suggested that repeat vaccination "effectively blocks the more robust, complex, and cross-protective immunity afforded by prior infection."
When unvaccinated people are infected with the seasonal influenza virus, they have an opportunity to develop a robust "cell-mediated" immunity that not only protects against that strain of the virus, but is also cross-protective against other strains. People who’ve annually received the influenza vaccine, on the other hand, "may have lost multiple opportunities for infection-induced cross-immunity."
This is because the vaccine is designed to stimulate a strong antibody response, or humoral immunity, but does not confer the same kind of robust cell-mediated immunity as natural infection.
In short, unvaccinated individuals who’d been infected with the seasonal influenza virus had also gained protection against the pandemic H1N1 virus, whereas the vaccine conferred no such protection.
That's a superb illustration of an opportunity cost of vaccination.
