ELI5 Vaccines - why is a Covid-19 vaccine is taking so long to make?

ELI5 Explain Like I'm 5: Bite sized answers to stuff you should know about - in a mini podcast

Why do some vaccines last longer than others i.e. some last for 10 years others only 1 year? What is the difference between an RNA vaccine and a conventional vaccine? Why does the process for making a vaccine take so long?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: ptk77, redshift2k5, fruitsausages, zoten, me_better, eatmyflakes & peppepcheerio

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