Around the world, several teams of scientists are looking to create "contagious vaccines", that would spread by themselves, giving herd immunity to populations faster than the disease can spread itself.
Vaccines that contain live or weakened virus have been shown to be contagious to a certain extent in the past, though data on how transmissible they are is scarce. One vaccine known to be contagious – the oral polio vaccine (OPV) – is one of the rare vaccines known to spread between people. When taken, the weakened virus replicates in the child's intestines, helping their immune system to build up antibodies, before the vaccine is excreted.
"In areas of inadequate sanitation, this excreted vaccine-virus can spread in the immediate community (and this can offer protection to other children through ‘passive’ immunization), before eventually dying out," the World Health Organization explain on their website.
Some scientists believe that deliberately making contagious vaccines for use in the animal population – for example in bats or other known reservoirs for zoonotic diseases – could be a way to fight these diseases before they ever have a chance to infect humans.
"Spillover of infectious diseases from wildlife populations into humans is an increasing threat to human health and welfare. Current approaches to manage these emerging infectious diseases are largely reactive, leading to deadly and costly time lags between emergence and control," one team from the University of Idaho wrote in a paper published in PNAS.
"Here, we use mathematical models and data from previously published experimental and field studies to evaluate the scope for a more proactive approach based on transmissible vaccines that eliminates pathogens from wild animal populations before spillover can occur. Our models are focused on transmissible vaccines designed using herpes virus vectors and demonstrate that these vaccines – currently under development for several important human pathogens – may have the potential to rapidly control zoonotic pathogens within the reservoir hosts."
However, the idea – as this team and others acknowledge – is not without its risks. Like "zoonotic disease", the main risk is a phrase you've become quite familiar with in the last few years: vaccine efficacy.
"Transmission has the benefit of increasing herd immunity above that achieved by direct vaccination alone but also increases the opportunity for vaccine evolution, which typically undermines vaccine utility," one team wrote in the journal Trends in Microbiology. Essentially, just like viruses (see the Alpha, Delta and Omicron variants and subvariants), the vaccine could evolve as it spreads, making it less like the disease it is vaccinating against, and decreasing efficacy, necessitating further vaccines.
The risks go beyond this, too, particularly should we ever use the idea to spread immunity in humans.
"The potential benefits of transmissible vaccines are enormous, but there are some safety oncerns that need to be figured out before their successful implementation," Mark Smithson of the School of Biological Sciences at Washington State University explained in a video.
"Use in humans may be warranted for populations that are hard to reach, or for epidemics that are uncontrollable by direct vaccination. However, using transmissible vaccines could be dangerous. Mainly because vaccines with a potential to spread through a host population also have the potential to revert back to the disease."
This isn't just a hypothetical, but something that has been seen with the oral polio vaccine.
"On rare occasions, if a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. In very rare instances, the vaccine-virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus (cVDPV)," the World Health Organization explain.
However, this is a problem that can be avoided.
"Circulating VDPVs occur when routine or supplementary immunization activities (SIAs) are poorly conducted and a population is left susceptible to poliovirus, whether from vaccine-derived or wild poliovirus. Hence, the problem is not with the vaccine itself, but low vaccination coverage. If a population is fully immunized, they will be protected against both vaccine-derived and wild polioviruses."
For now, the focus of creating contagious vaccines is centered on providing herd immunity to animals that are reservoirs of zoonotic diseases. Though potentially a gamechanger, the idea has so far only been tested once in practice.
Researchers captured 147 wild rabbits, before vaccinating about half of them against rabbit hemorrhagic disease and myxomatosis, before releasing all the rabbits – now microchipped – into the wild. Since the virus was similar enough to the original myxoma virus – which causes myxomatosis – the vaccine spread among the rabbits, and by the time they checked 32 days later, 56 percent of the non-vaccinated rabbits had antibodies to both viruses, suggesting some transmission of the vaccine.
Though risks will have to be monitored carefully, the benefits of this vaccination technique could be enormous. One mathematical model found that Lassa transmission rates in rats could be reduced by 95 percent over the course of three years.
As well as Lassa fever, self-spreading vaccines are currently being developed for Ebola and bovine tuberculosis, with hopes to soon target other zoonotic diseases.
If the technique proves to be successful and without harms, maybe the next potential pandemic could be one that we never end up hearing about.
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