The COVID-19 pandemic has exposed glaring gaps in the world’s current vaccine production capacities. Conventional vaccine manufacturing is costly and complex. As a result, only a handful of countries have the technology, human resources, and funds to make vaccines; those that are able have faced recurrent challenges of contamination and quality control in the race to manufacture and distribute billions of COVID-19 vaccines. Conventional vaccines also have to be kept cold, some as cold as -76 degrees Fahrenheit, during transport and storage. The vaccine cold chain is not only costly but is also a major barrier for vaccine distribution in rural, hard-to-reach communities and in countries with limited infrastructure. The solution, some scientists believe, is using plants to manufacture vaccines.
Although there are no plant-based vaccines available for human use, several are in the pipeline. Medicago, a Canadian biotechnology company, has developed a plant-based COVID-19 vaccine that is currently in phase three clinical trials. The company’s plant-based flu vaccine has completed clinical trials and is awaiting final approval from the Canadian government, according to Brian Ward, the company’s medical officer. In December, Kentucky BioProcessing, the U.S. biotechnology wing of the British American Tobacco Company, announced its plant-based COVID-19 vaccine was entering phase one clinical trials, and last October, Japanese-owned Icon Genetics GmbH launched phase one clinical trials for its plant-derived norovirus vaccine. Universities, biotechnology start-ups, and governments have formed well-financed partnerships to expand the field. The South Korean government has invested $13.5 billion toward plant-based vaccine research, and the country’s first plant-based vaccine production facility in Pohang city is expected to open in October. By one estimate, the plant-based vaccine market value is predicted to rise from $40 to $600 million over the next seven years.
“The plant-made vaccine industry has been moving slowly but surely forward. We’re at the point where making something like a vaccine to COVID is actually very feasible and very fast, so right now we are at the point where we can have tens of millions of vaccine available within, I’d say, the next six months or so,” says Kathleen Hefferon, an author and professor of microbiology at Cornell University, who specializes in plant research and agricultural biotechnology. “What I am really hoping is that this opens the floodgates for new advances in plant-made vaccine development, because now we will see some successes.”
The problems with traditional vaccines
Plant-based vaccine technology is not new; its proof-of-concept dates back about 30 years. Scientists have used potatoes, rice, spinach, corn, and other plants to make vaccines for dengue, polio, malaria, and the plague, but none of these were taken through to end-stage clinical trials, perhaps due to the lack of a regulatory framework for plant-based medicines or hesitancy around investing in emerging biotechnologies, according to Hefferon.
In 2006, the U.S. Department of Agriculture approved a plant-based vaccine for Newcastle disease, which infects poultry. But there has never been a plant-based vaccine approved for use in humans or, until recently, even one that made it to advanced clinical trials.
To make vaccines, scientists must mass produce antigens, molecules that trigger an immune response to a specific virus or bacteria. Common antigens include inactivated or killed viruses and bacteria, toxins, or viral and bacterial proteins such as the COVID-19 spike protein. For the Pfizer-BioNTech and Moderna COVID-19 vaccines, the mRNA molecules—the little pieces of genetic material with the instructions for human cells to manufacture the COVID-19 spike protein—must also be mass produced in expensive facilities and then purified.
Antigens for conventional vaccines are made by infecting laboratory-controlled cells (from insects, monkey kidneys, hamster ovaries, or others) with a virus or a bit of viral genetic code that tricks the cells to make copies of the virus or antigen. The cell lines are incubated in large, metal bioreactors for days to weeks, then undergo a lengthy and complex purification process before being packaged into vials.
The challenge is bioreactors are expensive, they require specially trained personnel to manage them, and the risk of contamination is high, so bioreactors growing different types of antigens must be kept in separate buildings and in tightly controlled, sterile conditions.
“We have seen in COVID that there isn’t enough manufacturing capacity globally to make vaccines for everyone,” says John Tregoning, an infectious diseases researcher at U.K.’s Imperial University. This is due to the prohibitive cost, space, and personnel requirements. The U.S. Department of Defense estimated that it costs $1.5 billion to maintain a facility that makes only three vaccines for 25 years.
Plants as vaccine factories
Plant-based vaccines eliminate the need for bioreactors because they themselves are the bioreactors. Plants can be grown in climate-controlled, pharmaceutical-grade greenhouses that keep out bugs and pests but do not require sterile conditions.
In Medicago’s greenhouse in Raleigh, North Carolina, two mechanical arms pick up a steel tray of 126 juvenile Nicotiana benthamiana plants, a weedy, Australian cousin of the tobacco plant used in cigarette products. The tray of plants is swiftly turned upside down and dunked into a metal basin of liquid containing millions of agrobacteria, a group of bacteria that naturally infect plants. The agrobacteria in this greenhouse are altered to contain a small piece of DNA from the influenza or COVID-19 virus. While the plants are submerged, a small vacuum sucks at the plant’s roots causing the leaves to collapse and shrivel up. A few seconds later, the vacuum is released, causing the leaves to re-expand and, like a sponge, soak up the liquid carrying the agrobacteria, which spread throughout the entire vascular structure of the plant.
In a matter of minutes, the Nicotiana benthamiana plants have been transformed into mini bioreactors. The agrobacteria transfer the viral DNA to the plant cells, which then make millions of copies of virus-like particles that serve as antigens but are not infectious.
“It’s totally cool. It’s one of the best things actually. It’s called agroinfiltration or vacuum infiltration,” says Medicago’s Brian Ward. The plants are resettled in the greenhouse and after five or six days, the leaves are harvested, placed on a conveyor belt, chopped into tiny pieces, and soaked in an enzyme bath that breaks down the hard plant cell wall and releases millions of virus-like particles, which are purified and packaged, Ward explains. The finished product is a plant-derived vaccine. In 2018, Medicago’s flu vaccine was the first in the world to complete phase three clinical trials.
For conventional vaccines, once the virus or viral particles are extracted from the cells that grew them and purified, they must be kept cold. This includes Medicago’s plant-based flu and COVID-19 vaccines.
But other plant-based vaccines skirt this problem by skipping the purification step altogether. Genetically modified lettuce is also commonly used to make vaccines. According to Henry Daniell, a researcher at the University of Pennsylvania who has been involved in lettuce-based vaccine work, scientists use a gene gun to insert a slice of viral DNA into the genome of a lettuce seed’s chloroplast, the part of the plant where photosynthesis—the process in which a plant converts sunlight into usable energy—takes place. Chloroplasts carry about 100 copies of their genome—the genetic material that provides the cell with instructions to function and make copies of itself—unlike most other cells, which have only one copy. This means chloroplasts can produce up to 100 times the amount of target antigen.
Once the viral gene has been inserted into the genome, the seed is grown under controlled but otherwise normal conditions at a farm or greenhouse and then harvested. But here, because lettuce is an edible plant, instead of purifying the virus-like particles by removing all the plant cells and debris, the chloroplasts containing the antigen are ground up into a powder that is then formulated into a pill or capsule, which would then be taken orally. Several lettuce-based vaccines for humans and animals are under development, but none have advanced to clinical trials. The advantage to a pill form of a vaccine is that it can be stored at room temperature for long periods, thus eliminating the cold chain problem.
Estimated costs to make plant-based vaccines are not yet publicly available, but according to Daniell, “There is no question that producing in plants instead of bioreactor is going to be cheaper. The bioreactor fermentation facilities are hundreds of millions of dollars, and then you need to purify and cold chain injection and so on.”
Emerging plant-based vaccine technology will not only help the world respond to the current and future pandemics but also offers an opportunity to expand vaccine production to developing countries, according to Hefferon. Vaccines remain a cornerstone of public health, preventing some 4 to 5 million deaths each year. And yet many places around the world lack access to vaccines for measles, meningitis, and whooping cough. That means that some 1.5 million people a year still die from preventable infectious diseases.
“There is an overwhelming vaccine inequality between the rich and the poor countries, and maybe if you can increase the amount of different manufacturing platforms, then you can make more vaccines more quickly for more people,” says Tregoning.