While vaccines for mumps, rubella and measles protect those who are vaccinated and prevent them infecting others, new jabs leave a risk of infecting others
New vaccines can spread killer diseases, say experts.
They say jabs for malaria, HIV and Ebola protect the patient but leave them contagious and at risk of infection to others.
Childhood vaccines for smallpox, polio, mumps, rubella and measles are classed as “perfect” because they protect those who are vaccinated and prevent them infecting others.
Prof Venugopal Nair, of The Pirbright Institute in Surrey, said the new “imperfect” vaccines can promote more virulent infections that survive and spread the disease.
But Prof Peter Openshaw, of Imperial College London, said: “It’s important not to interpret this as an argument against vaccination.”
British investigator Prof Venugopal Nair, of 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 un-vaccinated individuals at greater risk.”
Researchers found new types of vaccines can allow the recipient to experience mild symptoms and remain contagious, putting others at risk.
They pointed out that this sort of vaccine is used to control virulent strains of bird flu that could pose a potential threat to thousands of people.
It means imperfect vaccines can promote the evolution of more virulent and dangerous infectious agents that survive and spread disease, according to the study published in the online journal Public Library of Science Biology.
Scientists warned the findings have implications for the transmission of bird flu to humans, and vaccine strategies to control HIV, Ebola and malaria.
They confirmed the controversial theory after carrying out experiments with a type of herpes virus that infects chickens.
Researchers found that a “leaky” vaccine against Marek’s disease prevented treated birds from dying but allowed the virus to survive and kill un-vaccinated birds.
During the avian flu outbreak, infected poultry infected in the US and Europe were culled, but farmers in south-east Asia relied on “leaky” vaccines.
Study co-author Prof Andrew Read, of Pennsylvania State University in the US, said: “We humans never have experienced any contagious disease that kills as many un-vaccinated 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.”