https://www.nejm.org/doi/full/10.1056/NEJMoa2305478
Back in 2022, GSK was investigating a maternal RSV vaccine. However in February 2022, the independent safety monitoring group stopped the trial early. This was due to a higher rate of preterm birth in the vaccine arm compared to the placebo arm.
In this NEJM piece, the authors do not know the cause for the higher rate, nor if it is even a real vaccine risk. Of note, the higher rate was only seen during a particular time period in the trial suggesting a potential cofactor. It was also mostly seen in low and middle income countries not high income countries. Preterm birth didn't have a defined temporal association with administration of the vaccine either, suggesting the vaccine didn't have a direct effect on preterm birth.
So there could be potential confounders unrelated to the vaccine. The period of elevated risk was during the Delta wave which has been associated with higher preterm births. However, they did not see any association in the trial with coronavirus infection of the mother.
In both arms of the trial the rate of preterm births was below the background rate in most of the countries, further showing how small this risk signal was.
This isn't a concern with the approved Pfizer maternal RSV vaccine. But as another example of how cautious regulators are, the Pfizer maternal RSV vaccine is administered late in pregnancy just in case.
I'm always amazed at how anti-vaxxers talk about vaccines and the vaccine industry. In reality, vaccine research if anything takes safety extremely seriously. Such that a small safety signal can end a trial even if it may not be real.