What are cold chain requirements for vaccines and how are they maintained?

Cold chain maintains vaccines at recommended temperatures from manufacturing to administration. Requirements vary: standard vaccines (2-8C), some live vaccines need freezing (-15 to -25C), mRNA vaccines required ultra-cold (-60 to -80C for Pfizer, -25 to -15C for Moderna after formulation improvements). Cold chain components: validated refrigerators/freezers with continuous monitoring, temperature data loggers, insulated transport containers with coolants, cold boxes and vaccine carriers for last mile. Challenges include: power outages, equipment failure, temperature excursions during transport, last-mile delivery in developing regions. Solutions: solar refrigerators, freeze indicators, heat-stable formulations, dry ice or liquid nitrogen for ultra-cold. Temperature excursions may require vaccine discard or stability evaluation. WHO prequalification includes cold chain requirements.