Nearly every vaccine approved by the FDA introduces foreign proteins into healthy but nonimmune individuals. The immune system finds those proteins, which are often part of a pathogen (the measles virus, say), and then quickly learns to recognize them.
RNA vaccines inject not proteins but the molecules of nucleic acid that encode the instructions for building the proteins. Your cells use the RNA to instruct their builders to make the proteins.
An RNA vaccine injects instructions to your cells, and hopes that your cells receive these instructions and follow them, and build the proteins that will teach your immune system to fight a virus.
There are simply too many variables and way too many things that can go wrong. Despite some successes in animals, the RNA strategy has never yielded a vaccine approved for human use. It may be decades before we know the damage caused by such vaccines. https://www.theatlantic.com/ideas/archive/2020/03/two-extreme-long-shots-could-save-us-coronavirus/608539/
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