Imperfect Vaccines May Help Viruses Grow Stronger: Study

New research suggests that some vaccines, known as leaky vaccines, are imperfect and do not completely immunize against the disease, potentially turning the host into a transmission point for the disease. 

In a study published this week by the medical journal PLOS Biology, researchers examined the differences between so-called leaky vaccines and perfect vaccines in chickens infected with Marek’s disease, a herpes virus that infects chickens.

Leaky vaccines were found to potentially crease more powerful and deadly strains of the disease in some cases.

Researchers found chickens infected with the non-lethal or leaky strain of Marek’s, opposed to perfect forms of the vaccine, were able to transmit the disease over a long period of time after receiving the vaccine instead of simply becoming immune.

Typically, the perfect vaccine will offer immunity to the host and prevent them from transmitting the disease to others. In this case, the leaky vaccine did not affect vaccinated chickens, but it did allow them to transmit the disease to other chickens who will eventually die, unless they have been vaccinated.

“Vaccines that keep hosts alive but still allow transmission could thus allow very virulent strains to circulate in a population,” wrote study authors.

While the study focuses only on diseases in farm animals and does not prove the effect in humans, researchers say the same effect could potentially be seen in humans and could eventually cause serious pathogens to infect the population.

Immunizing the chickens with leaky vaccines also extended the infectious periods of the strains that were otherwise too lethal to live on in the vaccinated chickens for an extended amount of time. It also will wreak havoc on unvaccinated hosts, allowing them to become sick with the disease.

When vaccines work properly they prevent the transmission of the disease, like those used on humans, including the polio and smallpox vaccines, it prevents the diseases from becoming more virulent and dangerous.

“This idea follows from the notion that natural selection removes pathogen strains that are so ‘hot’ that they kill their hosts and, therefore, themselves,” said co-author of the study Venugopal Nair, of the Pirbright Institute in the UK. “Vaccines that let the hosts survive but do not prevent the spread of the pathogen relax this selection, allowing the evolution of hotter pathogens to occur.”

The results of the study did not directly account for the vaccine creating more powerful pathogens, like the overuse of antibiotics in creating drug resistant germs. Instead, the process was not a clear cause and effect.

However, the study did reveal a link between vaccines and the development of virus strains that became lethal to farm animals. Researchers say the real concern regarding human vaccines is the potential next-generation vaccines pose in creating virulent pathogens.

Researchers say the findings should be taken into consideration when vaccines are designed and when vaccine programs are implemented and monitored.