On: Pre-exposure to mRNA-LNP inhibits adaptive immune responses in mice

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"mRNA was in studies prior to COVID, its not like we made it up in 3 weeks."

You mean the animal trials in which mRNA based drugs repeatedly failed due to toxicity, for which as recently as 2017 there was no fix?

https://www.statnews.com/2017/01/10/moderna-trouble-mrna/

No, mRNA tech wasn't being used before COVID. They were trying to bring it to market and failing. That's why so many people refer to it as experimental.