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.
While poultry infected with avian flu in the US and Europe were culled, farmers in south-east Asia relied on leaky vaccines.