Interesting question. The vaccination occurs via im injection. Only a subfraction of it would get to the blood. The halflife of mRNA in the whole blood is restricted to seconds. But let's assume that it somehow manages to get to placenta. It would have to be actively transported to the fetal circulation and then get into the APCs and get transcribed and translated. It's a long and rather improbable way. But if that would somehow happen, then there would be a chance that the process you describe could occur.

I concur with Agnieszka; what you are concerned about is a very low probability event. The liposomes fuse to any cell they contact, so the odds of them surviving long enough to reach the placenta, much less pass through the placenta, are virtually nil.

There is one hook I must add: it is known that some maternal cells can cross the placenta into the fetus and persist for decades, and cells from the fetus can do likewise. This occurs at a very low but measurable level and is known as fetal:maternal microchimerism. This microchimerism can persist for decades, implying tolerance to the microchimeric cells. I have published on this.

So a transfected maternal cell MIGHT pass through the placenta, and MIGHT render the fetus tolerant. What is not clear is if this would be problematic; mice rendered tolerant to LCMV infection, which is lethal in mice, can be infected and survive.

As far as I know, they have not examined the effect of vaccinating pregnant women in a clinical study. I personally would recommend caution.