Vaccines usually leave the refrigerator for a few hours at a time: a nurse or health worker carries the day's doses from a health centre to a village session site, a school or an anganwadi. A vaccine carrier is a compact, insulated container designed for this trip. It contains no refrigeration system; instead, it is packed with ice packs that have been conditioned so they do not freeze the vaccines.
How well a carrier works depends on the quality of its insulation, how long the lid stays closed, whether it is kept out of direct sun, and whether the ice packs have been conditioned properly. Vials should not touch frozen ice directly. Most programmes give specific packing and handling instructions, and carriers used in national immunisation programmes are generally selected from models assessed against WHO PQS specifications.
An auxiliary nurse midwife in Madhya Pradesh might collect vials from the primary health centre's ice-lined refrigerator in the morning, pack them in a carrier with conditioned ice packs, and hold the session in a village two hours away, returning unopened vials as her programme guidelines direct.