Imperfect vaccines may increase transmission of highly virulent pathogens

Vaccines that let the host live but do not prevent transmission of the pathogen are called leaky or imperfect vaccines. In the early 2000s, it was proposed that such vaccines could lead to the emergence of highly virulent pathogens by creating ecological conditions that would allow the emergence of hotter and deadlier strains. 

A recent article published in July issue of PLos Biology, investigated this theory. In this study, the authors used Marek’s disease (MD) virus to show experimentally that immunization of chickens with a vaccine against MD virus can create conditions that promote transmission of lethal strains of the virus.  MD virus is a highly contagious oncogenic virus of poultry. At present, all MD virus vaccines are live viruses. The vaccinated birds can become infected and shed wild type viruses.

The authors performed a series of experiments to show that vaccination with MD vaccine increases the infectious period of the most virulent strain of MD virus by preventing the death of the host. As virulent strains have a deadly effect on the host, they are eliminated before infecting other hosts. The prolonged infectious period causes more shedding of the virus in the environment. They also showed that all the unvaccinated birds co-housed with vaccinated infected birds became infected and died.  Thus, vaccination allowed onward transmission of most virulent strains and could be risky for unvaccinated birds. 

The authors note that their data doesn’t show that vaccination caused the evolution of more virulent strains of MD virus. They further emphasize that despite the evolution of deadly strains, vaccination has greatly reduced the mortality due to MD viruses. Previous studies have observed that more virulent pathogens are overcoming more effective vaccines. A vaccine that not only protects host but also prevents onward transmission of the virus could be an answer to this problem.