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UNICEF Innovation

The Last Mile

The current problems experienced in cold chain transportation is the accidental freezing of vaccines. This Innovation project is looking to explore the possibility of utilizing Phase-Change-Material (PCM) to mitigate the risk of freezing.

© UNICEF/NYHQ2007-0480/Nesbitt


Over the last four decades, immunization programs have been transporting vaccines in insulated containers, which contain frozen water packs to maintain suitably cold temperatures. As of 2013, there are 16 vaccine carriers prequalified by WHO with capacities ranging from 0.80 L to 3.61 L. These carriers are generally used for the “last-mile” of vaccine delivery before it reaches the end-beneficiary. They can be carried by humans walking, on bicycles, or on motorbikes. 


This model carries the danger of freeze-sensitive vaccines losing their potency. If frozen water packs are improperly conditioned and/or vaccine refrigerators are malfunctioning, the internal temperature of the vaccine carrier can fall below the safe threshold to carry these vaccines. 

By mitigating this risk, wastage can be reduced and delivery be made more reliable for many vaccines including the following:

  • Diphtheria-tetanus-pertussis, and all diphtheria-tetanus-pertussis-containing multivalent vaccines
  • Tetanus toxoid
  • Diphtheria tetanus
  • Hepatitis B
  • Pneumococcal conjugate
  • Rotavirus
  • Human papillomavirus
  • Typhoid
  • Cholera
  • Inactivated polio vaccines


Phase-Change-Material (PCM) is a relatively new technology which is still in the development phase. 

It could offer the following advantages over traditional water-filled packs:

  • PCM packs do not require conditioning after freezing, reducing the risk of lost potency through freezing.
  • Autonomy: how much time elapses from when the PCM modules are put in the box, until the inside of the box reaches 8ºC. This can allow for longer vaccine deliveries before risking lost potency through overheating.
  • Weight: the boxes are often carried by the health workers over some distance, so the PCM solution should preferably be lighter than the water-filled packs used today.



The Technical Assessment which took place at UNICEF Supply Division has come to an end. After careful consideration, the project team has identified three Phase Change Materials (PCMs) out of six, which could be alternatives to water based ice packs. The next step of the project is to move to the field to test the three products.

The purpose of the field trial will be to:

  • Identify contenders for potential implementation of one product that is able to eradicate exposure to freezing, of temperature sensitive vaccines, utilizing the equipment that is already available in the field.
  • Measure the impact of reducing subjective conditioning of ice packs after freezing, making it safer for health workers to use packs directly from the freezer.
  • Measure time savings of human resources as a result of reduced or no conditioning.
  • Gather data on the potential loss of vaccines due to freezing and the impact of administering safe and potent vaccines to end users.

Country Offices will soon be invited to participate in the field trial and updates will follow once the field trial has been implemented



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