By Dr. Mercola

In 2001, Andrew Read, evolutionary biologist at Penn State University, and colleagues published a paper stating that imperfect vaccines, which protect the host from death but allow infection and disease transmission to other hosts, may actually trigger the evolution of more virulent forms of the disease.1
“Vaccines rarely provide full protection from disease,” the study’s abstract begins… and those that are imperfect or “leaky” may protect the host from succumbing to the disease while still allowing them to shed the virus into the community.
Ordinarily, a particularly lethal or “hot” virus is self-limiting because it will kill those it infects relatively quickly, before they have a chance to spread the virus to those around them. With leaky vaccines, this is no longer the case, as infected hosts may spread the lethal virus for weeks or more. In 2001, they wrote:2

“The subsequent evolution leads to higher levels of intrinsic virulence and hence to more severe disease in unvaccinated individuals. This evolution can erode any population-wide benefits such that overall mortality rates are unaffected, or even increase, with the level of vaccination coverage.”

The 2001 study was based on a mathematical model, and it was met with criticism and controversy, as are most studies that point out the imperfect nature of vaccinations or call for more research. Now, however, Dr. Read and his colleagues have further proof of their leaky vaccine theory, this time based on the spread of Marek’s disease in chickens.

‘Leaky’ Vaccines Can Enhance the Transmission of Highly Virulent Virus Strains

The researchers speculated that vaccines that keep hosts alive but still allow infection and transmission could allow very virulent strains of viruses to circulate in a population. This, they found, is precisely the case when chickens are immunized against Marek’s disease caused by a herpes virus that is often lethal


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