If the rate is so low in the US, why would it be unethical to do it there? You could also take samples from babies whose parents are getting or refusing the vaccine anyways. Sure, those decisions on their own will introduce more variables, but they could do a “outcomes of being raised by pro/anti vaccine/medicine parents” study.

If the goal was the truth, at least. It would be hard to do that study without any bias either way, with how political the whole thing is.