US pharmaceutical firm Pfizer says its coronavirus vaccine should be frozen for transport in principle, and refrigeration should only be an option when freezing is impossible.
Pfizer Japan on Monday issued guidelines on handling vaccines for local governments and healthcare workers.
Japan is to start giving shots to the elderly on April 12.
The guidelines say the vaccines must not be subjected to shock or vibrations because they break down easily, and they should be frozen when distributed in small batches to vaccination facilities.
The guidelines say local governments should only transport vaccines at temperatures from two to eight degrees Celsius when they cannot be frozen.
Pfizer had told Japan's health ministry last December that refrigerated transport is not recommended but would be acceptable if there is no other option.
The ministry notified local governments this month that freezing the vaccine will ensure better quality control.