In various countries, teams of scientists are looking for ways to create "infectious vaccines". This vaccine is designed to spread on its own and establish herd immunity in a population faster than the spread of the disease itself.
Vaccines containing live or attenuated viruses have been shown to be contagious to some extent, although data on modes of transmission are scarce.
There is one vaccine that is known to be contagious, namely the oral polio vaccine (OPV), which is one of the rare vaccines known to spread between humans. When taken, the attenuated virus will replicate in a child's gut, helping their immune system to build antibodies, before the vaccine is issued.
"In areas with inadequate sanitation, this released vaccine virus can spread in nearby communities, and it can offer protection to other children through 'passive' immunization, before eventually dying," the World Health Organization said in a statement on its website. they.
Quoted from IFL Science, some scientists believe that deliberately developing infectious vaccines for use in animal populations, for example in bats or other zoonotic disease reservoirs, could be a way to fight these diseases before they can infect humans.
"The spillover of infectious diseases from wildlife populations to humans is an increasing threat to human health and well-being. Current approaches to managing emerging infectious diseases are largely reactive, leading to a deadly and costly time lag between emergence and control," the team wrote. researchers from the University of Idaho in a paper published in PNAS.
The research team explained that they used mathematical models and data from previously published experimental and field studies to evaluate the scope of a more proactive approach based on infectious vaccines that remove pathogens from wild animal populations before spillovers can occur.
"Our model focuses on infectious vaccines designed using herpesvirus vectors and suggests that these vaccines, currently under development for several important human pathogens, may have the potential to rapidly control zoonotic pathogens in reservoir hosts," they explain.
Risk of infectious vaccine
But the idea, as this team and others admit, is not without risk. As with zoonotic diseases, the main risk is a phrase we have become familiar with in recent years: the efficacy of vaccines.
"Transmission has the benefit of increasing herd immunity above that achieved by direct vaccination alone. But on the other hand it also increases the chances of vaccine evolution which usually undermines the utility of vaccines," one of the team wrote in the journal Trends in Microbiology.
Basically, just like viruses (eg Alpha, Delta and Omicron variants and subvariants), vaccines can evolve as they spread, make them less favorable to the disease they are vaccinated against, and reduce their efficacy, thus requiring further vaccines.
The risks also go beyond this, especially if we ever use the idea of spreading immunity in humans.
"The potential benefits of a transmissible vaccine are enormous, but there are several safety concerns that need to be addressed before successful implementation," said Mark Smithson of the School of Biological Sciences at Washington State University.
"Human use may be necessary for populations that are difficult to reach, or for epidemics that cannot be controlled by direct vaccination. However, the use of vaccines that are transmissible can be dangerous. Especially because vaccines that have the potential to spread through the host population also have the potential to return to disease."
This is not just a hypothesis, but something that has been seen and proven with the oral polio vaccine.
"On rare occasions, if a population is severely under-immunized, the released vaccine virus can continue to circulate for a long period of time. The longer it is allowed to persist, the more genetic changes it undergoes. In very rare cases, the vaccine virus can genetically change into a form that can paralyze. This is what is known as the circulating vaccine-derived poliovirus (cVDPV)," explained WHO.
For now, the focus of making infectious vaccines is focused on giving herd immunity to animals that are reservoirs of zoonotic diseases. Despite the potential to be groundbreaking, the idea has so far only been tested once in practice.
The researchers captured 147 wild rabbits, before vaccinating about half of them against rabbit hemorrhagic disease and myxomatosis. The rabbit is then released into the wild.
Because the virus was quite similar to the original myxoma virus that causes myxomatosis, the vaccine was spread among the rabbits, and by the time they examined them 32 days later, 56% of the unvaccinated rabbits had antibodies to both viruses, indicating some transmission from the vaccine.
Although the risks must be carefully monitored, the benefits of this vaccination technique can be enormous. One mathematical model found that Lassa transmission rates in mice could be reduced by 95% over three years.
In addition to Lassa fever, self-propagating vaccines are currently being developed for Ebola and bovine tuberculosis, with the hope of targeting other zoonotic diseases soon.
If this technique proves to be successful and harmless, perhaps the next potential pandemic can be prevented in this way so that we will never again experience such a thing as a pandemic.