mRNA is very unstable at room temperature in the presence of oxygen, so it is unlikely to survive long. The environmental stability of the Spike protein, which is what the ribosomes in our body produce from the mRNA, will depend a ton on the conditions. Temperature, acidity, UV light, and many other factors will influence whether the protein maintains its configuration and thus its function. Still, the dose of spike must be fairly high to have an effect on a person, and must bypass the body's innate defenses. Without the ability to produce more, the effect will be quite transient.