"Cold Defence" nasal sprays are recommended to be taken either preventatively or in the early stages of a cold. The active ingredient in these sprays is Carrageenan. After some research, the active compound is Iota-Carrageenan (ι-Carrageenan).
This study by Eccles et al shows that this drug is clinically effective. The study was only designed to test efficacy rather than to explain the mechanism, however. A fleeting mention is provided in the discussion section:
The above results suggest that the treatment with Iota-Carrageenan reduces the viral replication. Consequently fewer cells are infected, the immune reaction against the viruses is less pronounced and fewer symptoms occur.
How does ι-Carrageenan produce an anti-viral effect?
- Eccles R, Meier C, Jawad M, Weinmüllner R, Grassauer A, Prieschl-Grassauer E. 2010. Efficacy and safety of an antiviral Iota-Carrageenan nasal spray: a randomized, double-blind, placebo-controlled exploratory study in volunteers with early symptoms of the common cold. Respiratory Research, 11:108, doi:10.1186/1465-9921-11-108.