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Insulating materials that could fit inside icepacks to transport and store vaccines more effectively are about to enter field trials in Vietnam.

The novel materials make use of the phase change — the point at which solids melt or liquids turn to solids — to keep the vaccines within a limited temperature range and prevent them from spoiling because of temperature variations.

If successful, the new vaccine carriers could be produced in India for use around the world, according to Shawn McGuire, an engineer at the global non-governmental organisation Program for Appropriate Technology in Health (PATH).

Vaccines are only effective if they remain within a narrow temperature range, often between two and eight degrees Celsius. But when a health worker opens a refrigerator door, it changes the temperature inside.

In a hot climate with unreliable electricity, icepacks are often used in the vaccine supply chain to prevent overheating.  

But ice is colder than vaccines need to be, and improperly stored vaccines can freeze and fail to protect patients.

"This has become more of an issue, as most new vaccines are freeze-sensitive," said Osman David Mansoor, a senior adviser on new vaccines at the UN Children's Fund (UNICEF), in New York, United States. "A single freeze exposure can inactivate some vaccines."  

Currently health workers have no easy way of knowing whether a vaccine has frozen and later thawed out.

Previous studies have shown that vaccines are often exposed to freezing conditions, as a result of ice, more frequently during transport than storage, according to Mansoor.

The phase-changing materials integrated into vaccine-carrying boxes act "like a shield around the vaccine" said Nancy Muller, PATH programme officer.

These materials absorb or release large amounts of heat by changing phases between liquid and solid states — just the way water does.

However, the temperature at which they do this — their melting point — can be above zero degrees Celsius, at a temperature appropriate for the safe transport of vaccines.

Used in combination with ice, such materials could provide additional temperature stability for vaccines, said Ian Tansley, founder and chief technical officer at the UK-based engineering company True Energy, which also uses the technology for fridges that can keep cool longer without electricity.

PATH expects its prototypes, which would fit into existing WHO-approved ice-carrying boxes, will cost more than existing coolers.

But over the product's lifetime — which can reach a decade — it should save enough vaccines to make up for it.

"An ice-pack that did not expose vaccines to freezing conditions would indeed be an important advance," said Mansoor.