Compounds that could stop coronavirus and flu viruses from infecting human cells have been discovered in sea sponges.

Researchers unearthed 26 such compounds found in nature, including in plants and fungi.

The international research team say their discovery paves the way for new "natural" medicines and antivirals than can help treat contagious viruses.

They claim the compounds will help tackle existing and future variants—as well as flu—because they target human cells, which evolve more slowly than viruses.

They are effective in very small doses in the lab, where the compounds completely stopped viral infection in human cells.

"The advantage of these compounds is that they are targeting the cells, rather than the virus, blocking the virus from replicating and helping the cell to recover," explained Dr. Jimena Pérez-Vargas from the University of British Columbia in Canada, who co-authored the study.

For the study, which was published in the journal Antiviral Research, the team investigated more than 350 compounds derived from natural sources including plants, fungi, and marine sponges, in a bid to find new antiviral drugs that can be used to treat the novel disease—or in 26 cases, completely stop coronavirus infection in cells.

They bathed human lung cells in solutions made from these compounds and then infected the cells with Covid variants.

The researchers used a version of the coronavirus which causes cells to go bright green when they are infected, as well as a special screening technique, to make the discovery.

They say the fluorescent virus is a powerful tool that enable scientists to check thousands of compounds, track the virus as it moves from one cell to another and makes extremely laborious steps that used to be necessary completely redundant.

All three of the most effective compounds were found in Canada: alotaketal C, from a sea sponge in Howe Sound, British Columbia; bafilomycin D from a marine bacteria in Barkley Sound, British Columbia; and holyrine A from marine bacteria collected in waters off Newfoundland.

Further tests showed the three compounds were effective against the delta variant and several omicron variants.

Bafilomycin D was found to work synergistically with an existing antiviral nasal spray against omicron sub-variant BA.2. This paves the way for multi-drug treatments that work against the coronavirus as well as other common viruses.

The team now want to test the compounds in animal models in the next six months.

The study's senior author, UBC's Dr. François Jean, added: "Our research is also paving the way for large-scale testing of natural product medicines that can block infection associated with other respiratory viruses of great concern in Canada and around the world, such as influenza A and RSV."

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