With mRNA vaccines, producing "next up" vaccines in a very small timeframe is easy. Once they create one, creating the next set is easy. That's similar to, but not exactly the same as, how flu vaccines can be produced in less than a year. They don't know what next flu season's vaccines are going to be like until they see the new flu viruses. They have to guess, somewhat, but they also have to churn out an entire vaccine series in less than a year.