They should either be added to the transport medium when the samples are ready to be collected or added during preparation of the media which should then be frozen at -20 °C and thawed only when needed.
If attempts to isolate virus are to be made, transport media containing specimens should not be
stored at -20°C but at -70°C or in a liquid nitrogen storage unit.
Storage at -20 °C is adequate if the specimen is to be used for PCR tests only.