Check out this great video
Vaccines require strict Temperature control in order to be effective. If they get too hot, they no longer work, if they get too cold, they lose their efficacy.
This challenge can be met with technology called PCM (Or phase change material boxes) which are paraffin based wax panels that are pre-conditioned, and can maintain 2-8 C for 4-6 days. This is the story about how one Regency in Sulawesi is using this technology to reach those on the last mile.