Smoking was once deemed safe and effective for pregnant women, because it relaxed them.
Smoking was never medically proven to be safe or effective for pregnant women, though there was a period in the mid-20th century when some doctors informally suggested it for certain conditions like weight control or anxiety.
If, in the future, something better and cheaper than mRNA technology comes along, mRNA will get "kicked to the curb" as it should. Until that happens, medical scientist should explore expanding its use.
Just banning pregnant tourist will not solve the problem. It will certainly mitigate it but it will not solve it. People will just come and stay longer; perhaps on a H1B visa. To really solve the problem, you have to address the reasons that bring them here in the first place and condition the benefits accruing from those reasons on their contributions "over time" to the sources that enable those reasons to exist.