Generally speaking, CDC recommends avoiding the top shelf and the areas near vents due to temperature fluctuations. However, most purpose-built or pharmaceutical-grade units use a fan to circulate air within the storage area and create more uniform temperatures than household units. During a power outage, the top shelf is an area of caution for all units as the temperatures increase most quickly there. In this instance, it would be best to check with the manufacturer or owner’s manual to see if the top shelf is appropriate for storage in your unit. Units that meet the NSF/ANSI 456 voluntary certification standard are designed to deter the user from placing vaccines in areas where proper storage temperatures cannot be maintained, so any shelf available storage in a certified unit would be usable.