Vaccines are powerful, but they are not perfect. In some cases, communities struggle to vaccinate enough individuals to stop the spread of a pathogen.
But suppose that instead of vaccinating most of a population, it were possible to vaccinate just a few individuals. In theory, a benign yet infectious vaccine could effortlessly and silently pass protection from one individual to another.
In a new mathematical model, researchers demonstrate that a weakly transmissible vaccine significantly lowers the incidence of infectious disease and paves the way toward eradication. The work was published October 26th in the journal Proceedings of the Royal Society B.
Researchers have long been interested in the idea of transmissible vaccines, and the idea has becoming increasingly viable over the last five years, says Leor Weinberger, a virologist at the University of California, San Francisco, and the Gladstone Institutes, who was not involved in the research. For example, several recombinant, transmissible vaccines are in development for wild animal populations, including one to protect wild rabbits against a fatal viral infection and another to prevent deer mice from carrying a virus responsible for a deadly human pulmonary disease.
But any discussion about a transmissible vaccine ultimately comes down to risk—in particular, the risk of the vaccine reverting to a pathogenic virus. This occurred accidentally with the oral polio vaccine in the early 1960s and again in the 2000s: On a few, rare occasions, the vaccine—a live, attenuated strain of poliovirus—reverted to virulent, neuron-attacking strains of the virus and caused polio infections.
“Obviously this is a controversial and potentially risky endeavor, so we wanted to figure out, is the potential benefit actually worth the risk?” says study co-author Scott Nuismer, a professor of biological sciences and mathematics at the University of Idaho.
Nuismer, with collaborators at six other institutions, created a mathematical model based on the standard paradigm for how a vaccine spreads through a population. But he and the team altered a key factor—a vaccine’s ability to spread. How much transmissibility among individuals would be required to significantly lower a population’s incidence of an infectious disease?
The answer was not much. “If I’m being sincere, I was surprised how well it works,” says Nusimer. “Just a little bit of transmission had substantial gains.” Even a weakly transmissible vaccine—where only one out of every three vaccinated individuals transmits the vaccine to another individual—was able reduce the number of infected individuals in a hypothetical population by 61%, and resulted in disease eradication in 39% of disease simulations.
“It’s always good to see more confirmation” of the viability of transmissible vaccines, says Weinberger. He notes, however, that the model does not take into account a heterogeneous population in which some individuals spread the disease more often than others. Nuismer hopes to include such complexity in future models; he suspects a transmissible vaccine would actually perform even better in such an environment, as the ability to percolate through “superspreaders” would likely increase a vaccine’s transmission potential.
But don’t expect to see a transmissible vaccine for humans anytime soon. For human applications, the risks still outweigh the benefits. The most likely near-implementations will be vaccines to protect wild animal populations or livestock from pathogens, says Nuismer. The ability to vaccinate a few animals—rather than capturing and vaccinating most of a population—could save agricultural and environmental agencies millions of dollars and countless hours of animal capture and release.
In the pursuit of such vaccines, several teams are developing genetic engineering methods to create vaccines protected against reversion. One option is to build vaccines that are only conditionally alive in the presence of the pathogen. Weinberger’s lab has been developing small, “therapeutic interfering particles” that compete with a pathogen for resources in the cell. Such particles are engineered to replicate only in the presence of a pathogen, and to spread between individuals—a transmissible therapy with no chance of reversion.
“It’ll be a little while,” says Weinberger, “but there are a number of groups working on technology to try to realize these [vaccines].”