New single-dose, temperature-stable rabies vaccines could expand global access
A pair of gloved hands holds a syringe and a vial with a vaccine solution. (Credit: Adobe Stock)
Roughly 60,000 people worldwide die every year from rabies, a dreaded virus that attacks the nervous system and can trigger aggression, seizures, paralysis and coma.
In industrialized countries, infections and deaths in humans are rare, thanks to vaccines widely given to pets and people for prevention and available as a life-saving treatment once someone has been exposed. But in developing countries, including rural parts of Asia and Africa, rabies remains a major threat.
Now, CU Boulder researchers have discovered a new way to make human rabies vaccines that could greatly expand access to immunization across the globe. The new method, outlined in an August 2025 paper in the Journal of Pharmaceutical Sciences, creates shots that are temperature-stable—meaning they don’t need to be stored at cold temperatures like traditional rabies vaccines.
These innovative shots also combine multiple timed-release doses into a single injection, potentially reducing the number of health care visits each person needs and helping to break down barriers to care. The same process could also be used to create other vaccines, including those for human papillomavirus (HPV) and human immunodeficiency virus (HIV).
“We think the implications of this are huge,” said Ted Randolph, a professor in CU Boulder’s Department of Chemical and Biological Engineering and the lead author of the new study. “We’re really excited about it.”
Challenges of current rabies vaccines
Vaccines can work in a variety of ways. Some, like vaccines against flu or rabies, expose the body to weakened, inactivated or killed viruses. This teaches the body to recognize proteins found on their surfaces and create antibodies that fight future infections by binding to those proteins. Others, like protein-based vaccines for Covid, contain select proteins from the target pathogen that can trigger a similar immune response.
From left, Ted Randolph and colleague Robert Garcea pose for a photo. (Credit: Glenn Asakawa)
All currently marketed vaccines need to be kept refrigerated or frozen—sometimes at temperatures as low as minus 76 degrees Fahrenheit—because the proteins in them start to degrade at warmer temperatures.
Like milk that sat out on the counter too long, a vaccine solution can curdle as its proteins break down and clump together. At that point, the shots are no longer effective. Cooling them slows down the protein degradation process, said Randolph.
“The proteins basically want to make cheese,” he said. “You have to keep them from making cheese for long enough that you can manufacture the vaccines, get them to pharmacies and hospitals, and get them to patients.”
This makes it difficult, if not impossible, to administer traditional rabies vaccines in regions that lack electricity or don’t have the specialized cold storage equipment needed. In areas with electricity but poor infrastructure, a single power outage can wipe out vaccine supplies for entire communities.
The rabies vaccine also requires between three and five doses at timed intervals, depending on the patient. People in developing countries tend to visit doctors less often and have a harder time accessing medical care, so they are less likely to get all the needed doses.
‘Sapphire-coated Jolly Ranchers’
Even at warm temperatures, the shots developed by Randolph’s team don’t degrade.
To make them, the team sprays sugar solutions containing inactivated rabies viruses and other vaccine components through nozzles that make a fine mist, which dries to form a powder of microparticles. These microparticles have a glassy texture similar to that of a hard candy. The rabies virus proteins are immobilized and preserved in the candy coating, like ancient insect fossils trapped in amber.
Next, the team coats the candied particles with a layer of aluminum oxide (sapphire) of precisely controlled nanoscopic thickness using a process called atomic layer deposition pioneered by Alan Weimer and Steven George, professors in engineering and chemistry at CU Boulder.
Because sapphire dissolves very slowly once injected into a patient, the nanoscopic sapphire layer protects the sugar-coated vaccine particles for days to weeks, depending on the thickness of the sapphire layer applied on the microparticles. When the sapphire starts to break down, the sugar layer dissolves, and the vaccine particles are released into the body one dose at a time.
A microscopic image shows vaccine particles with a sapphire coating. These particles are fractured to show the coating. (Credit: Ted Randolph)
“We're basically making sapphire-coated Jolly Ranchers,” Randolph said.
These vaccines are stable at high temperatures, can be stored in a dry powder form and delivered in bulk to parts of the world that lack cold-storage capacity.
“You can now take these vaccines to places without refrigeration, and even to places that get hot,” Randolph said. “So transportation through rural India or wherever you're going is no longer a problem.”
It’s too soon to know how effective these vaccines are in humans. Currently, they’re being tested in animals, and human clinical trials are at least a couple of years away. But the results from early testing have been promising.
In mice, the researchers found that even single injections of the spray-dried, sapphire-coated vaccine powders sparked stronger immune responses than multiple doses of traditional liquid rabies vaccines. The immune responses did not weaken after storing the vaccines for three months at temperatures up to 104 degrees Fahrenheit.
Randolph and his colleague Robert Garcea, professor emeritus in CU Boulder’s Department of Molecular, Cellular and Developmental Biology, have formed a startup company called VitriVax to bring the technology—decades in the making—to market.
“It's been 25 years of lots of talented grad students adding little bits and pieces to the puzzle. It’s the kind of thing that does require long-term dedication, work and funding,” Randolph said.