While the world considers whether to give toddlers the COVID-19 vaccine, scientists are raising concerns about the potential risks that “leaky” vaccines pose to both children and adults.
A new study published in the scientific journal PLOS Biology reports that some vaccines may enable more virulent versions of a virus to survive, which puts the unvaccinated at a higher risk of severe illness.
To understand what is happening, it is necessary to look at the difference between “perfect” vaccines and “leaky” vaccines. Perfect vaccines are called this name because they mimic the natural immunity that humans develop after having certain childhood diseases like polio, smallpox, and more.
“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.
Studies with chickens
Professor Read was examining vaccines that could protect against malaria when he began to study Marek’s disease, which is a very contagious viral disease that infects chickens.
“It’s a form of herpes that is found in chicken dander and is more virulent than the Ebola virus,” Read said.
Researchers at The Pirbright Institute in the United Kingdom conducted experiments in a specialized pathogen-containment facility. They concluded that the Marek’s disease vaccines were 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 vaccines have a leaky barrier against the virus. Those who have been vaccinated may get sick, but they will experience less severe symptoms. However, the virus survives long enough to transmit to other individuals, which means the virus will survive and spread through the community.
“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.
It’s important to consider the history of Marek’s disease in chicken and the vaccine developed to address it. Marek’s disease was previously a minor illness that did little harm to chickens in the 1950s. However, due to the vaccine, the virus is more virulent and able to kill all of the unvaccinated chickens in a flock, frequently within 10 days of an initial infection.
But since nearly every chicken in agricultural production throughout the world is vaccinated, Marek’s is a relatively minor problem today.
Preventing more-virulent virus strains
Marek’s disease is not the only example. The virus that causes avian influenza can be even more deadly.
“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,”
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 the evolution of the avian influenza virus toward greater virulence could happen,” he added.
What’s even more concerning is that some human deaths due to avian influenza 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.
How should science and the medical industry respond? There must be robust testing and watchful monitoring of future vaccines to prevent the creation of more virulent strains of viruses. 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 the prevention of malaria.