A study of chickens suggests leaky vaccines allow viruses to evolve and increase the risk of more severe disease to unvaccinated individuals.
Many vaccines - such as the childhood vaccines against smallpox, polio, measles, mumps and rubella - are perfect. Because they mimic the way the immune system works, they not only stop vaccinated individuals from getting the disease, they also stop it spreading to others.
But some vaccines are "imperfect" or "leaky" in that while they protect the vaccinated individual from getting sick, they do not prevent the transmission of the pathogen, which is able to survive and spread throughout a population.
This ironically gives pathogens a chance to evolve that is denied them in a vaccine-free world. In the vaccine-free world, really "hot" pathogens die out because they kill their hosts.
The new study is the work of an international group led by Andrew Read, a professor in biotechnology at Penn State University, and Venugopal Nair, a professor who heads an avian viral disease program at the Pirbright Institute in the UK. It was Prof. Read who first raised the idea of leaky vaccines, a theory that was met with much skepticism when it was first proposed 10 years ago.
For their study, published in the journal PLOS Biology, the team carried out experiments with various strains of a herpes virus that causes Marek's disease in poultry. The disease causes great distress to poultry - it attacks the nerves and causes paralysis and widespread tumors.
Leaky vaccines allow 'hotter' strains to evolve
The results confirmed that birds vaccinated against the disease with modern vaccine can shed the virus and pass it on. The team found that some of the milder Marek's disease strains killed around half of unvaccinated birds within two months, but the more virulent strains killed 100% of them in much less time.
In unvaccinated flocks, the more virulent or "hot" strains burned themselves out because they killed their hosts quickly before they had time to shed and spread the virus.
However, when the researchers tested the virulent strains in flocks containing vaccinated and unvaccinated birds, they found the vaccinated birds had more time to shed virus and pass it on because they survived.
Prof. Nair concludes:
"Our research demonstrates that the use of leaky vaccines can promote the evolution of nastier 'hot' viral strains that put unvaccinated individuals at greater risk."
Prof. Read says he is concerned about the next generation of vaccines for human diseases. If some of these prove to be leaky, they could spur the evolution of more virulent strains.
He also notes that the study offers strong evidence in favor of individuals getting vaccinated, because as strains evolve and become more virulent, it is the unvaccinated who are at the greatest risk. "Those who are not vaccinated will be exposed, without any protection, to the hottest strains of a virus," he explains.
Meanwhile, Medical News Today recently reported that European drug regulators have approved the world's first malaria vaccine. The European Medicines Agency recommend the vaccine RTS,S (brand name Mosquirix) be given to children in Africa aged 6 weeks to 17 months. Children in Africa form the majority of the 600,000 victims that the mosquito-borne disease kills every year.