It's difficult to pin down "when did mRNA research begin?" but, a pretty good date is to say, "The 1990s." But you could go back as far as 1960 or 1970 if you were being technical.
https://publichealth.jhu.edu/2021/the-long-history-of-mrna-vaccines has a good write-up.
mRNA technology is a HUGE breakthrough. Like I said, we were lucky it was essentially ready and able to help with the COVID vaccine development when it was.