Imperfect Vaccines Can Enhance Virulence of More Pathogenic Viruses Making it More Dangerous for Those Not Vaccinated
“Vaccines are safe and effective”, is the sound bite you are all familiar with. But we know from past experience (the old whole cell DPT for example) that not all vaccines were safe, some still in use and some being heavily promoted (like the HPV – Human Papilloma Virus) carry substantial risks of harm to otherwise healthy teens and preteens getting that vaccine. We know from the experience with the Prevnar (vaccine for pneumococcal infections) that the initial Prevnar covered 7 strains and that then led to new, more virulent strains taking over. They responsed by making a Prevnar 13 to cover those additional strains. Will a new, more virulent strain emerge?
The article “Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens,” showed by experimentation that “immunization of chickens against Marek’s disease virus enhances the fitness of more virulent strains, making it possible for hyperpathogenic strains to transmit.” You can read this article here… Authors discuss the phenomenon of leaky vaccines where a vaccine may prevent some disease but the host goes on to transmit other diseases, that may be even worse than the one prevented by the vaccine. They state, “our data show that anti-disease vaccines that do not prevent transmission can create conditions that promote the emergence of pathogen strains that cause more severe disease in unvaccinated hosts.” While this study was in animals, it highlights the need for caution and long term surveillance when a new vaccine is introduced to the population. The HPV might be a perfect example of a vaccine that may ultimately result in greater virulence of other strains. We’ve already seen the vaccine expanded to cover more strains and it remains to be seen whether the overall impact will be the creation of new more virulent strains.
This issue of vaccines enhancing virulence and making it more dangerous for unvaccinated individuals may require a different line of thinking. One where it is not the unvaccinated that are endangering the vulnerable, but rather it is the vaccination putting the vulnerable at risk. I’m in favor of ongoing study and long-term study of the population effects of our vaccine programs. We must keep an open mind to risks and benefits.