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 to chickens.³

In unvaccinated chickens, the most virulent strains of Marek’s disease burn out on their own because the hosts are killed quickly and have little time to spread the disease.

However, when the researchers tested the virulent virus strains in vaccinated birds, they found they were infected and survived but shed the virus for much longer and therefore passed it on to far more birds. They wrote:

“Immunity elicited by direct vaccination or by maternal vaccination prolongs host survival but does not prevent infection, viral replication, or transmission, thus extending the infectious periods of strains otherwise too lethal to persist.

Our data show that anti-disease vaccines that do not prevent transmission can create conditions that promote the emergence of pathogen strains that cause more severe disease in unvaccinated hosts.”

This is added proof that long-term use and overuse of vaccines could present real problems for individual and public health by ultimately allowing highly virulent diseases to spread and persist in the community.