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Scientists are developing controversial 'contagious vaccines' that could jump from vaccinated to unvaccinated, but experts warn of potential dangers

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Ajpek Orsi/Getty Images

Scientists are currently attempting to develop "contagious vaccines" that would spread from the vaccinated to the unvaccinated, according to a new report.

A vaccine that fights disease but is also contagious is in development around the world. Scientists believe that creating a contagious vaccine could help prevent pandemics started by animals. However, some experts note that introducing a self-spreading vaccine is not only controversial, but also potentially dangerous.

Researchers are developing genetically engineered viruses that can spread from one animal to another, which will provide immunity to the disease. National Geographic reports that scientists are working on contagious vaccines for the extremely deadly Ebola virus, bovine tuberculosis, and Lassa fever. All three are zoonotic diseases – which are infectious diseases that are naturally transmissible from animals to humans.

Scientists believe they could expand the development of self-disseminating vaccines to other zoonoses such as rabies, West Nile virus, Lyme disease, and the plague.

"Zoonotic pathogens may be bacterial, viral or parasitic, or may involve unconventional agents and can spread to humans through direct contact or through food, water or the environment," the World Health Organization states. "They represent a major public health problem around the world due to our close relationship with animals in agriculture, as companions and in the natural environment. Zoonoses can also cause disruptions in the production and trade of animal products for food and other uses."

The Centers for Disease Control and Prevention notes, "Zoonotic diseases are caused by harmful germs like viruses, bacterial, parasites, and fungi. These germs can cause many different types of illnesses in people and animals, ranging from mild to serious illness and even death. Animals can sometimes appear healthy even when they are carrying germs that can make people sick, depending on the zoonotic disease."

The CDC says that 60% of all known infectious diseases and 75% of new or emerging infectious diseases are zoonotic.

The researchers are targeting zoonotic diseases because wild animals are extremely difficult to vaccinate, but a self-spreading vaccine could immunize large populations of wildlife.

A paper published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) in January states, "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."

"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," write the authors of the paper from the University of Idaho. "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, there are some major concerns about the possible dangers of contagious vaccines.

A Popular Science article from 2017 warns about viruses mutating, "If we did intentionally design transmissible vaccines, they might be more likely than regular vaccines to revert. That's because they reach more people and have a chance to replicate and make new generations. That means more chances for mutations and evolution."

"Then your transmissible vaccine turns back into the disease effectively," says Scott Nusimer, a mathematical biologist at the University of Idaho.

As the disease evolves, new variants of the disease could emerge, which will significantly decrease the efficacy of the contagious vaccine.

The outlet claims that you could make the vaccine "only weakly transmissible," but it would spread minimally before dying out, and it wouldn't be able to eradicate a disease.

A 2018 paper cautions that you should "expect evolution to drive the vaccine back closer to its wild-type phenotype," and that "revertants will be a minor contribution to all infections."

"By contrast, even infrequent reversion of an attenuated vaccine will preclude its use against a not-yet-present infectious disease where it would have the undesirable consequence of introducing the disease it was designed to block. And reversion thwarts the final steps of eradication," the paper reads.

The oral polio vaccine (OPV) is one of the rare vaccines known to spread between people, but the WHO warns there have been some issues.

"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 WHO explains. "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 paralyze – this is what is known as a circulating vaccine-derived poliovirus (cVDPV)."

"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," the WHO says. "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."

Jonas Sandbrink –a biosecurity researcher at the University of Oxford’s Future of Humanity Institute – tells National Geographic, "Once you set something engineered and self-transmissible out into nature, you don't know what happens to it and where it will go. Even if you just start by setting it out into animal populations, part of the genetic elements might find their way back into humans."

There is a potential risk that the contagious vaccines could disrupt natural population control, which could cause pests to explode in numbers – potentially altering the ecosystem and posing a threat to crops.

Sandbrink cautions that self-spreading vaccines also present a biosecurity threat in which bad actors could alter genetic stability with techniques that “uniquely advance certain capabilities applicable to the creation of viruses for pandemics and as biological weapons."

There are also ethical and consent issues with self-spreading vaccines for humans.

"We can't even get people to take a vaccine in a global pandemic. The idea that you would be able to surreptitiously vaccinate the population with a virus without causing riots is just, you know, it's stuff of fantasy. It will never be used in humans," says Alec Redwood – a principal research fellow at the University of Western Australia.

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