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Depends on the virus.

Coronavirus is an "easy" one to create a vaccine against. HIV would be the toughest one, regular approaches don't work, which is why we don't have a vaccine despite 30 years of trying.

For HIV you need a different approach, called "germ-line targetting", where instead of presenting the virus, you are trying to direct the evolution of antibody producing cells towards a specific kind which is able to produce working antibodies. If you just present the virus, you get useless antibodies.

So if we are hit by a "hard" virus like HIV, mRNA vaccines will be useless, since it takes years to figure out that germ line targeting.