By studying chickens, researchers say they have proven the theory that more virulent viruses can evolve from so-called “leaky” vaccines.
The current debate over vaccinating toddlers is small potatoes compared to the potential risks of using “leaky” vaccines to prevent disease.
According to a new study published today in the scientific journal PLOS Biology, some types of vaccines could allow more virulent versions of a virus to survive, putting those who are unvaccinated at greater risk of severe illness.
To understand this, it’s necessary to examine the difference between “perfect” vaccines and “leaky” ones.
Perfect vaccines are so-named because they mimic the perfect immunity that humans naturally develop after having certain childhood disease.
“When a vaccine works perfectly, as do the childhood vaccines for smallpox, polio, mumps, rubella and measles, it prevents vaccinated individuals from being sickened by the disease, and it also prevents them from transmitting the virus to others,” said Andrew Read, an author of the study and an Evan Pugh professor of biology and entomology and Eberly professor in biotechnology at Penn State University.
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Enter the chickens.
Read was studying vaccines that could combat malaria when he got interested in Marek’s disease, a highly contagious viral disease that affects chickens.
It’s a form of herpes that is found in chicken dander and is more virulent than the Ebola virus, Read said.
After experiments done in a specialized pathogen-containment facility at The Pirbright Institute in the United Kingdom, the researchers concluded that the vaccines developed to combat Marek’s disease were imperfect or leaky.
“These vaccines also allow the virulent virus to continue evolving precisely because they allow the vaccinated individuals, and therefore themselves, to survive,” said Venugopal Nair, who led the research team. He is the head of the Avian Viral Diseases program at The Pirbright Institute.
These less-than-perfect vaccines create a “leaky” barrier against the virus. Vaccinated individuals may get sick but have less severe symptoms, but the virus survives long enough to transmit to others, which allows it to survive and spread throughout a population.
“Our research demonstrates that the use of leaky vaccines can promote the evolution of nastier ‘hot’ viral strains that put unvaccinated individuals at greater risk,” Nair said.
Marek’s disease used to be a minor ailment that did little harm to chickens in the 1950s, but the virus has grown stronger and today is capable of killing all the unvaccinated birds in poultry flocks, sometimes within 10 days.
But since nearly every chicken in agricultural production throughout the world is vaccinated, Marek’s is a relatively minor problem today.
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Marek’s is not the only nasty disease out there. The virus causing avian influenza can be even deadlier.
“The most virulent strain of avian influenza now decimating poultry flocks worldwide can kill unvaccinated birds in just under three days,” Read said, because the vaccine against avian influenza is a leaky one.
“In the United States and Europe, the birds that get avian influenza are culled, so no further evolution of the virus is possible,” Read said.
Culling is a more expensive process than using a leaky vaccine, he said. But it’s safer.
“Instead of controlling the disease by culling infected birds, farmers in Southeast Asia use vaccines that leak — so evolution of the avian influenza virus toward greater virulence could happen,” he added.
Some human deaths from avian influenza virus have been reported in China.
“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,” Read said.
What’s the answer?
Rigorous testing and vigilant monitoring of next-generation vaccines to prevent the evolution of more-virulent strains of viruses will help.
Read sees this as crucial to the current attempt to develop an Ebola vaccine. He notes that secondary techniques can help when using leaky vaccines, such as insecticide-treated bed nets for prevention of malaria.
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