'Leaky' vaccines, as they're known, don't offer complete protection, and the vaccinated person can pass on the virus to others. And because the virus survives the vaccine, it can develop into a more virulent form.
The theory that vaccines cause more virulent strains of a virus was suggested around 10 years ago, and scientists from Penn State University have finally been able to confirm it.
They noticed the phenomenon in vaccines for Marek's disease, a condition that afflicts poultry. The disease was a relatively benign disease in the 1950s, but today it can kill all unvaccinated birds within 10 days.
The researchers fear the same thing is now happening with the avian flu virus. The vaccine is 'leaky', and the virus is now killing unvaccinated birds in just three days. Although infected birds are culled in the West, farmers in South-East Asia are turning to vaccination instead, and a more deadly virus is now evolving. This also has serious implications for humans if they become infected.
Prof Andrew Read at the university fears that we could be seeing a similar pattern with the HIV and malaria viruses if a 'leaky' or less-than-perfect vaccine is developed.
He stresses that common vaccines, such as the MMR, are 'perfect' and do not make the virus more virulent.
(Source: PLOS Biology, 2015; 13: e1002198)