ASF Vaccine Development

Wednesday, December 29, 2021

Scientists at secure, high-containment laboratories around the world are continuing to work and share information to develop a vaccine that will protect against the deadly swine virus.

By Jackie Clark

Over the past year and a half, most of the world has been focused on the COVID-19 pandemic and the progress toward developing a safe and effective vaccine. However, for those in the pork industry, a parallel effort is underway to find a vaccine to protect against African swine fever (ASF).

ASF has no clinical impact on humans, but it is devastating to swine herds, and has spread across domestic and wild pig populations in Asia, Africa, Europe, and most recently the Dominican Republic.

A handful of laboratories around the world with specific biosafety level designations and high-containment facilities are working on developing ASF vaccines.

In Canada, that includes the National Centre for Foreign Animal Disease (NCFAD) in Winnipeg and the Vaccine and Infectious Disease Organization (VIDO-InterVac) in Saskatoon.

"VIDO is pretty new to the game, having just received federal approval to work with ASF here in our facility under high containment," Dr. Andrew Van Kessel tells Better Pork. He is the associate director of research at VIDO-InterVac.

"NCFAD scientists are involved in evaluating the efficacy of ASF vaccine candidates," explains a spokesperson for the Canadian Food Inspection Agency (CFIA). CFIA collaborates with VIDO-InterVac on Canada's ASF preparedness strategy, including the vaccine testing effort.

Internationally, the United States Department of Agriculture (USDA) heads up the Global African Swine Fever Research Alliance (GARA), and includes ASF experts from around the world, including Russia, China, South Africa, the United Kingdom, France, the Netherlands, Sweden, Uganda and Spain.

"This group is tasked with focusing on ASF and one of their big goals is to evaluate what gap in knowledge exists in terms of vaccination in an effort to coordinate all the different groups working in that area," Dr. Lisa Becton, a director of animal health for the National Pork Board, tells Better Pork. "This really, truly is a global effort."

Scientists engage in information sharing which helps to ensure duplication is minimized, so that time, energy and resources are not wasted. Private industry partners are also collaborating on the effort.

The U.S. ASF vaccine effort is centered at Plum Island Animal Disease Center in New York.

Vaccine candidates developed there "have been licensed by the USDA to several commercial partners worldwide," say Dr. Manuel Borca and Dr. Douglas Gladue. They are both USDA-ARD research biologists for foreign animal disease research. "Currently most of the commercial partners are undisclosed, but we do have a cooperative agreement that is public – with Navetco in Vietnam – to help produce an ASF vaccine there."

Officials in "Vietnam are working very closely with USDA to look at some of the vaccine candidates that USDA has developed and actually field trial them in a country where the disease is present," Becton says.

Because so few labs can work with ASF, "there is frequent communication in terms of methods and approaches that could ultimately lead to the solution," says Van Kessel.

Two approaches

Scientists are "looking for two major outcomes from an ideal vaccine," explains Van Kessel. Firstly, the vaccine should provide protection from clinical signs, including mortality. In addition, the vaccine should eliminate the ability of the virus to replicate, so that vaccinated pigs do not shed the virus.

One approach to those goals is "a form of gene editing. That is, manipulating parts of the virus to alter infectivity, while still allowing it to react with the immune system," Becton explains. "If we can mimic an infection without all the negative impacts, such as viral shedding or damaging clinical signs and pathology, we can get that into the pig and confer the best immunity to it."

Researchers "are using the virus itself and deleting genes in the virus that are responsible for pathology, clinical symptoms. Those are called deletion mutants that lack virulence, so that when we expose animals to those viruses, they don't cause any disease because they lack the genes to do so. Secondly, though, they still induce an immune response so when the animal is exposed to the virus they are protected from the disease," explains Van Kessel.

This is one of two main types of vaccines that scientists are focused on.

Another type of vaccine being explored is a vector delivered vaccine, he adds. "We're working on an adenovirus vectored vaccine."

Scientists must "identify specific genes in the ASF virus, move those genes from that virus into an adenovirus, and deliver them to the pig as a vaccine. Our challenge is to identify the antigens, the proteins in ASF that, if we move them over into an adenovirus and expose the pigs to it, it will elicit an immune response against those proteins that will actually protect against the ASF virus challenge."

Challenges

Applying those approaches in practise has proved to be no easy task.

"ASF is a large DNA virus with a complex pathogenicity (the capacity of a microbe to cause damage). Over the years, many researchers have failed in their attempts to produce a vaccine that induces protective immunity safely," says the statement from CFIA. "The role of the majority of ASF virus proteins (the potential vaccine targets) is unknown."

Scientists in the U.S. are up against the same obstacles.

"ASF is a large virus with around 180 genes, of which most are not functionally characterized," Borca and Gladue explain. "This complicates the rational development of live attenuated vaccines since it is difficult to identify the genes involved in virulence."

Van Kessel agrees. "ASF is quite complex, it's a double-stranded DNA virus," he says. "It takes very sophisticated, high-containment facilities to study it, so we don't know a whole lot about most of those genes."

Also, "the specific proteins involved in the induction of protective immunity are unknown, which makes developing a subunit vaccine a very hard task. All attempts to use a small subset of proteins for vaccination have not resulted in an effective vaccine against highly virulent strains," Borca and Gladue add.

Finding target proteins is made more difficult because of the size of the virus and multiple viral strains, Becton explains. "Just because you create a vaccine for one strain doesn't necessarily mean that it's going to protect against another. Researchers are looking at the basic characteristics of the virus such as the mode of action and understanding how the virus actually gets into cells to infect a pig to try to identify unique but stable proteins that could be altered but still provide protection against infection."

Another challenge "is the ability to grow the virus in quantities sufficient to produce a vaccine," Dr. Paul Sundberg, executive director of the Swine Health Information Center, says. Prior to recent developments, "the only way to grow ASF was in pig white blood cells."

This method helps to keep the virus stable but complicates research.

"The best way to (grow ASF) in order to avoid unplanned changes in the virus, is to grow it in macrophages that we actually harvest from a pig," says Van Kessel. Immortal cell lines (cells that proliferate indefinitely) would allow for faster large-scale growth of ASF, but in the cell lines we have currently "the virus tends to be unstable."

Scientists observe changes to ASF genetics when growing it this way, which "creates challenges when you're trying to study the function of each of these genes," he explains.

The lack of immortal cell lines also presents a challenge for gene deletion vaccines, he adds. "How do you grow that virus now for commercial production? Using macrophages is not a commercially viable approach."

However, with a live attenuated vaccine, "if we're successful in finding those genes that are protective and moving them into an adenovirus, systems for growing adenovirus vaccines on a commercial scale are all well-developed," says Van Kessel. "Once you're successful, the commer-cialization pathway is pretty clear."

Another potential advantage of a viral vector vaccine over a deletion mutant vaccine is safety, says Van Kessel. Using a deletion mutant, "there's always the risk of a return to virulence."

Successes

Scientists have overcome some challenges in their work on developing ASF vaccines.

In testing early candidates, some pigs had a hyperimmune response, says Sundberg. "The pigs that were vaccinated died faster than the pigs that were not vaccinated. That's an immune allergic response."

Currently, USDA prototypes are being tested in the field in Vietnam, he adds. These new candidates "don't have the challenge of hyper-immune reaction that earlier vaccines did."

The USDA announced in May 2021 that some candidate ASF vaccines had been adapted to grow in a continuous cell line.

"Up until recently, all of these vaccine candidates required primary swine cells for production. Recently we adapted some of our vaccine candidates to grow in cell culture, which will aid in the production of the vaccine in a larger scale," explain Borca and Gladue.

In an additional step forward, several USDA "vaccine candidates have been removed from the select agent list," they add. This means they can be produced in Biosafety Level 2 facilities – which are more common than facilities with higher biosafety levels.

Scientists at NCFAD have also developed several attenuated vaccine candidates, and observed that some provide protection against ASF in experiments, says CFIA. "Live attenuated vaccines are promising candidates for ASF and are being tested in ASF affected countries. This work is currently underway."

What would vaccination look like?

Having a safe and effective vaccine is one piece of the puzzle. The other is a vaccine strategy.

If we have a vaccine before we see any cases of ASF in North America "the vaccine would most probably be used as a response tool," says Sundberg. "I would not expect the vaccine to be used in the industry to prevent infection because, depending on the type of vaccine, that may cause trade issues."

If the virus had not yet reached Canada or the U.S., vaccination would likely be response-based rather than preventative, he explains.

That strategy would depend on "the ability to establish a DIVA vaccine," says Van Kessel. DIVA stands for differentiation of infected and vaccinated.

With DIVA vaccines, industry stakeholders would have a way to tell if antibodies in a pig were produced from a vaccine or a wild-type virus, Sundberg says.

The DIVA process would differ by vaccine type.

"When you immunize with adenovirus with a single or a couple of ASF genes, it's easy to develop a test to say that pig was exposed to two ASF genes, but not any other genes, therefore that pig has never been infected with ASF," Van Kessel explains. "You can do DIVA with a deletion vaccine, but it's a bit more difficult."

The DIVA distinction would be "important in terms of business continuity and maintaining our trading partners," he adds. "We would not use a vaccine in Canada or the U.S. unless we were able to clearly differentiate the fact that the pig was vaccinated, and never exposed to ASF."

Another important issue to consider is administration, Becton explains. The process would be simple in commercial pig production, but "how do you manage feral pigs or wild boar populations?"

Scientists would need a vaccine that is stable enough to be administered through alternative methods and not injection, she says.

Impact

If ASF doesn't reach Canada or the U.S., a vaccine is still an important outcome to work toward.

"While the routine use of an ASF vaccine in the absence of an outbreak would not be anticipated at this time, an available vaccine for use in an outbreak situation would be a valuable tool in our response toolbox, to lessen the impact and duration of the disease," says a statement from CFIA.

The timeline of vaccine development and predictions of ASF spread are still uncertain, and much effort is still being put into prevention and treatment.

"We are much further along in the process than we were several years ago," Becton says. "However, it will take, probably, several more years at the very minimum to be able to get a product under development into commercial production."

When ASF first became a problem, some experts predicted "a 10-year cycle before we have a safe and effective vaccine," says Sundberg.

"The progress that's been made in the last three years since we've had ASF in China has really been remarkable. Also, that testing of vaccines in the field in Vietnam is a big step."

He thinks that we could see a commercial vaccine in the next few years.

"We cannot comment on a specific timeline, as there are many factors involved that are out of our control," say Borca and Gladue.

"It's impossible for us to predict the regulatory requirements that will be necessary in different countries. We will let our commercial partners make those predictions."

Steps toward success give pork producers around the world hope.

"ASF is causing a pandemic, with most of the focus on outbreaks in Europe and Asia. However, ASF has also been a problem in Africa for many years, with recurring outbreaks," Broca and Gladue explain.

"An effective vaccine would be helpful as part of any global control or eradication strategy, particularly for farmers and countries that cannot implement strict biosecurity measures."

Researchers here at home are working hard to help.

"It is important that Canada provides our experience and resources to work on a vaccine to help protect against a potential ASF outbreak in future. and contribute to stopping the spread of ASF globally," says CFIA. BP