Viruses are kept cold for the same reason we refrigerate food items. “You wouldn’t take a steak and leave it out on your counter for any length of time and then eat it,” said Caryn Heldt, director of the Health Research Institute at Michigan Technological University and professor of chemical engineering. “A steak has the same stability issues — it has proteins, fats, and other molecules that, in order to keep them stable, we need to keep them cold.”

Like proteins, viruses unfold when it’s hot or there’s space to move around. Heat provides energy for viruses to shake themselves apart, and not being crowded gives them the room to fall apart. Stable vaccines need cold or crowding.

But what if cold storage isn’t available? What if someone accidentally leaves the package on the counter? What if the power goes out?

Heldt, together with Sarah Perry, professor of chemical engineering at the University of Massachusetts Amherst, has developed a way to mimic the body’s environment in vaccines using a process called complex coacervation. Rather than relying on refrigeration, Perry and Heldt tap the other method to keep viruses stable — crowding.

Freezer Camp

To keep the viruses in vaccines stable, everyone along the supply chain, from manufacturing facility to shipping company to doctor’s office, must maintain the cold temperature. This cooperative effort is known as the cold chain. If a vaccine is kept above that temperature range for even an hour, it may become ruined and unusable.

The World Health Organization estimates that up to 50% of vaccines are wasted every year because the cold chain and ideal temperature for storage cannot be maintained.

The human body is a crowded place. Cells of varying shapes and sizes jockey for position. This includes viruses, which do their nefarious work by hostile takeover. Viruses invade our cells, commandeering them to replicate. Unchecked, virus copies explode out of the cells like darts through a balloon. Then all of those replicas go and do the same to other cells — and before you know it, you’re sick.

Heldt researches vaccine manufacturing techniques and the COVID-19 pandemic has served as a masterclass. But SARS-CoV-2 isn’t the only virus in the world — there is still need for other vaccines and storage methods that don’t rely on refrigeration.

“The conditions for a vaccine that make it good to be injected into someone’s body are almost the opposite of what makes a virus stable,” Heldt said. “There’s a really hard trade-off of keeping the virus stable to get good immune response, while having the right components in the vaccine that are safe to inject.”

Virus Burritos

Heldt and Perry use polypeptides — synthetic proteins — that have positive or negative charges. When these charged peptides are put in solution, they stick together and form a separate liquid phase, a process called complex coacervation. The liquid wraps around virus capsids, holding the virus material together like a burrito’s tortilla.