'Imperfect' vaccines can spread disease
Imperfect vaccines can promote the evolution of more virulent and dangerous infectious agents that survive and spread disease, a study has shown.
The findings, reported in the online journal Public Library of Science Biology, are said to have implications for the transmission of bird flu to humans, and vaccine strategies to control HIV, Ebola and malaria.
Scientists confirmed the controversial theory after carrying out experiments with a type of herpes virus that infects chickens.
They found that a "leaky" vaccine against Marek's disease prevented treated birds from dying but allowed the virus to survive and kill unvaccinated birds.
British investigator Professor Venugopal Nair, from The Pirbright Institute near Woking, Surrey, said: "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."
Childhood vaccines for smallpox, polio, mumps, rubella and measles are said to be "perfect" because they both protect vaccinated individuals and prevent them infecting others.
But other types of "leaky" vaccine can allow the recipient to experience mild symptoms and remain contagious, said the researchers.
This sort of vaccine is used to control virulent strains of bird flu that pose a potential threat to humans, they pointed out.
While poultry infected with avian flu in the US and Europe were culled, farmers in south-east Asia relied on leaky vaccines.
Study co-author Professor Andrew Read, from Pennsylvania State University in the US, said: "We humans never have experienced any contagious disease that kills as many unvaccinated hosts as these poultry viruses can, but we now are entering an era when we are starting to develop next-generation vaccines that are leaky because they are for diseases that do not do a good job of producing strong natural immunity - diseases like HIV and malaria."
He added that it was critically important to be sure the Ebola vaccines now in clinical trials are not leaky.
"We do not want the evolution of viral diseases as deadly as Ebola evolving in the direction that our research has demonstrated is possible with less-than-perfect, leaky vaccines," said Prof Read.
Professor Adrian Hill, director of the Jenner Institute at Oxford University, said: "The suggestion that altered pathogen virulence should be looked for after vaccinating people is not new, and there is no evidence that human vaccines have produced more virulent pathogens.
"So, it is important not to claim from this example in chickens that this is a problem with vaccines in humans. There is strong evidence that widely used human vaccines are not producing any such effect."
Professor Peter Openshaw, from Imperial College London, who is president of the British Society for Immunology, said: "It's important not to interpret this study as an argument against vaccination of our children against flu or any other disease.
"The standard vaccines that are in current use are safe and effective, and not prone to cause the emergence of more dangerous strains of viruses."