Most infectious diseases that have emerged to afflict humans in the last decade have come from animals. Ebola and the new coronavirus likely originated in bats. Developing vaccines to protect humans from these pathogens can take years and cost billions of dollars.
Why wait for an epidemic? We should vaccinate animals before viruses can jump to humans, argues Scott Nuismer, a professor of biology at the University of Idaho. Nuismer uses computer modeling to research how vaccines move through animal populations. “What we’ve been really pushing for is switching the whole paradigm about how we go about tackling this problem,” he says.
Nuismer and a handful of other research teams want to develop vaccines for animals that spread with the same voracity as viruses. Instead of passing on disease, the vaccine would spread immunity. But scientists will have to overcome some challenges first.
The problem: Vaccinating wildlife
Wildlife managers typically immunize animals via injection or vaccine-laden bait. Inoculating animals by injection is time-consuming and expensive, though. It took two years for the government of Wales to vaccinate 2,776 of about 61,000 badgers for tuberculosis, at a cost of roughly $2.3 million. Using bait is cheaper, but not all animals at risk will eat the vaccinated bait.
The solution: Vaccines that spread like viruses
Scientists including Nuismer, a team in Spain, and Michael Jarvis, a virologist from Plymouth University in the United Kingdom, have been researching the feasibility of genetically engineering vaccines that target Ebola in apes, hemorrhagic fever in rabbits, and Lassa virus in a common African rodent known as the multimammate rat.
“Identifying which of these coronaviruses poses the greatest risk is perhaps a greater challenge than building the transmissible vaccine itself.”Scott Nuismer, a professor of biology at the University of Idaho
Transmissible vaccines for animals would be made by engineering a benign virus to carry extra genetic material. Once administered cheaply to an animal via injection, the harmless virus would propagate, using the extra genetic instructions to grow structures that make it look like a disease-causing pathogen. The virus would stimulate the inoculated animal’s immune system to create antibodies or white blood cells that recognize those structures. But not only that: because it’s a virus, it also would “infect” nearby animals of the same species — through saliva, for instance.
Once it makes that leap, scientists say, the viral vaccine would immunize the nearby animals as well. When the real disease strikes, the population of animals that “caught” the vaccine would be protected.
Nuismer thinks he and his fellow researchers could have a proof-of-concept vaccine in the lab for Lassa virus within the next two to five years, but it would need regulatory approval before field trials.
The challenges: The vehicle, the vaccine, and the virus
It’s important to find a benign virus that can be genetically modified to work as a vaccine delivery vehicle. A herpes virus called cytomegalovirus (CMV) may be a good candidate, says Nuismer, because many animals, like rodents and bats, carry it, but it does little harm. It’s also unique to each species, which reduces the chances that a vaccine developed for bats, for instance, might accidentally transmit to birds. Nuismer says that experiments done on lab mice indicate that CMVs propagate quickly through a population. But scientists haven’t yet confirmed whether a CMV-based vaccine would actually jump from animal to animal in the wild.
A genetically engineered viral vaccine would also need to be stable. Viruses mutate as they move through a population. An engineered virus may shake off the extra genetic baggage after a few generations and revert to its former self. This could serve as a natural safety switch, preventing evolution from producing something potentially dangerous. But if it shuts down too soon, the vaccine won’t reach the entire population of animals, says Nuismer.
Targeting coronaviruses, such as the one that causes COVID-19, could be especially challenging, because a lot of different coronaviruses circulate among many different animal populations. “Identifying which of these coronaviruses poses the greatest risk is perhaps a greater challenge than building the transmissible vaccine itself,” says Nuismer.
The big picture: Protecting humans indirectly
It’s possible that transmissible vaccines could someday confer immunity to humans, but the science is a long way away. Besides, ethical considerations might prevent it from ever happening. A self-disseminating vaccine would spread to people who hadn’t agreed to be immunized. “It sounds like a mess to me,” says Nuismer.
But if scientists can develop the technology to stop infectious diseases in animals before they jump to humans, they could save many lives. It’s only a matter of time before the next sweeping pandemic emerges from the wild.