Most vaccines require refrigeration to keep from going bad. The messenger RNA vaccines for COVID-19 require freezing temperatures.


From the cutting-edge vaccines for COVID-19 to the decades-old ones for poliovirus, most vaccines need to be kept cold to survive the trip from factory to patient. But that poses a major hurdle to even routine immunizations in countries like Mali or Bangladesh, where up to 90% of health facilities lack adequate refrigeration. To solve this problem, some researchers are working toward a radical goal: vaccine formulations that don’t have to be kept cold. Significant hurdles remain, but many scientists are optimistic that 10 years from now vaccination campaigns won’t be quite so hampered by the heat.

“I think we are at the limit of how many people we can vaccinate using [refrigerated supply chains],” says Asel Sartbaeva, a chemist at the University of Bath who is working on a molecular “cage” to make multiple vaccines temperature stable. “And this is where we come in.”

Most vaccines include biomolecules or weakened forms of pathogens that start to fall apart above—or below—certain temperatures. Even when the formulas are freeze-dried or suspended in solutions to improve stability, many still require refrigeration between 2°C to 8ºC, the temperature of a regular fridge. When vaccines don’t have to be kept cold, the results can change the world: A freeze-dried smallpox vaccine that was stable for months at high temperatures was essential for eradicating the global scourge in the 1970s. Even incremental improvements can make a big difference: MenAfriVac, a meningitis vaccine that can now be kept without refrigeration for 4 days, cut costs in half during a 2011 vaccination campaign in Chad.

Scientists have different strategies for creating vaccines that can beat the heat, ranging from modified chemical solutions to new methods of delivery. Which strategy works best depends largely on the vaccine’s active ingredient: RNA, DNA, live or inactivated virus or bacteria, or bits of the pathogen like peptides or proteins.

Protein-based vaccines, which are relatively stable, are “low-hanging fruits,” says Maria Bottazzi, a microbiologist who co-directs the Texas Children’s Hospital Center for Vaccine Development. But other types, like live attenuated viral vaccines, are especially susceptible to changes in temperature. Diverse approaches are necessary, she says: “There’s no silver bullet.”

But there are silica cages. That’s what Sartbaeva used to stabilize proteins in the common vaccine for diphtheria, tetanus, and pertussis (DTP). Because the DTP vaccine is made of proteins from each of the disease-causing microbes, it needs to be kept at 2°C to 8ºC. Sartbaeva, whose previous research focused on the structure of porous silicate materials, thought silica might be able to create a protective molecular cage around the vaccine’s proteins, preventing them from unfolding in the heat.

Over the next few years, Sartbaeva and colleagues developed a process in which reactive silica molecules are mixed with the proteins. Attracted to the biomolecules’ positively charged regions, the silica comes together in a network that precisely matches the proteins’ contours. “The cool thing about silica is that it’s quite malleable,” allowing for a perfect fit, Sartbaeva says.

Her group’s silica cages stabilized diphtheria and tetanus proteins for at least 1 month at room temperature and 2 hours at 80ºC, she and her colleagues reported last year in Nature Scientific Reports. When injected into mice, the caged proteins provoked an immune response; the uncaged proteins were duds. But several challenges remain before clinical trials are in sight: proving the method can stabilize all the proteins in the DTP vaccine at once and simplifying the process needed to dissolve the silica before the vaccine is injected.

Another approach is the “Fruit Roll-Up” method. Maria Croyle, a pharmacist at the University of Texas, Austin, encased live adenovirus—a vector used in multiple vaccines, including Johnson & Johnson’s COVID-19 vaccine—within a solid film of sugars and salts with a texture similar to the popular snack. The film can be dissolved under the tongue or inside the cheek to administer the vaccine, or even reconstituted and injected. The film kept an adenovirus-based vaccine for Ebola stable at room temperature for 36 months, Croyle and colleagues reported last year in Science Advances. When reconstituted from the film and inhaled, the 3-year-old viruses protected primates from a lethal dose of Ebola.

The researchers add that by altering the mix of sugars and salts in the film, the method could work for other vaccines and therapeutics, including influenza vaccines. Croyle, who is also chief scientific officer of the biomedical startup Jurata Thin Film, says her colleagues there want to start clinical trials for a vaccine using the film within 12 to 18 months, once they determine how to scale up manufacturing. Michigan Technological University chemical engineer Caryn Heldt calls the film a “very promising technology” for multiple vaccine platforms. Bottazzi agrees, and says she’s considering testing the formulation with some vaccine prototypes.

Meanwhile, refrigeration-dependent messenger RNA vaccines—like those developed by Moderna and the Pfizer/BioNTech collaboration for COVID-19—present their own challenges. Some scientists have turned to a tried-and-true approach used with other vaccines: freeze-drying. Once a vaccine is freeze-dried, says Drew Weissman, an immunologist at the University of Pennsylvania, “It’s essentially stable forever. … The big challenge is the cost.”

New manufacturing techniques could help, but freeze-drying is a slow and expensive process, and it doesn’t work for every vaccine. Just this month, however, Pfizer started clinical trials on a freeze-dried version of its vaccine designed to be stable at regular refrigerator temperatures; currently, it has to be stored at about –20ºC. Results are expected at the end of May.

Another strategy: Alter the lipid nanoparticles that surround the vaccines’ RNA molecules; it is these nanoparticles that are responsible for the vaccines’ ultracold storage requirements. Seattle-based HDT Bio recently took this tack, inventing a nanoparticle that can be shipped in regular refrigerators, then combined with RNA just before injection, says Amit Khandhar, the company’s chief of formulations. The approach doesn’t entirely eliminate the need for refrigeration, and the more stable formulation does not provoke as much of an immune response, Weissman says. But the world may soon know how well it performs in the wild: A vaccine candidate for COVID-19 using the company’s nanoparticles is about to start clinical trials in India.

To avoid cold storage entirely, another option is to use DNA instead of RNA. DNA, the more stable of the two nucleic acids, “could be sitting in a warehouse at room temperature for months,” says Deborah Fuller, a microbiologist at the University of Washington, Seattle, who helped design the HDT Bio vaccine. DNA vaccines have not had much clinical success despite years of research, Fuller says, though a number of DNA vaccines for COVID-19 are in clinical trials.

But even if technical hurdles are overcome, getting temperature-stable formulations to market requires “aligning all the stars,” for manufacturers and regulators, Bottazzi says. Encouraging drug developers to spend millions of dollars on new formulations is no easy task. Incentives are rare, especially when it comes to reformulating existing vaccines, says Jason Hallett, a chemist at the Future Vaccine Manufacturing Research (FVMR) Hub at Imperial College London who works on temperature-stable vaccine formulations using liquid salts. Under current regulations, “It is more cost effective to improve the cold chain than it is to eliminate the cold chain,” he says.

Yet the enormous resources that countries have poured into COVID-19 vaccine research could change the outlook for temperature-stable formulations, says Benjamin Pierce, operations manager at the FVMR Hub. A temperature-stable vaccine may not come in time for the current pandemic, he says, but as the world prepares for the next one, “eliminating the cold chain will absolutely be a priority.”

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