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

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 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]