I remember reading somewhere a claim that the initial development of the (a?) mRNA vaccine was basically over a weekend; the testing and scaling to manufacture took the rest of the time.
True; it was ready in January. Proving it is safe definitely takes times.
> By the time the first American death was announced a month later, the vaccine had already been manufactured and shipped to the National Institutes of Health for the beginning of its Phase I clinical trial.
A big part of the safety concerns were about the lipid layer outside the mRNA triggering autoimmune reaction, which doesn't need to change, so changing only the mRNA data should be relatively safe.
> By the time the first American death was announced a month later, the vaccine had already been manufactured and shipped to the National Institutes of Health for the beginning of its Phase I clinical trial.
https://nymag.com/intelligencer/2020/12/moderna-covid-19-vac...