The NHS informs concerned parents about vaccines:
The 4-in-1 pre-school booster is very safe. Before it was granted a licence, the safety, quality and effectiveness of the pre-school vaccine, like all vaccines, was thoroughly tested. It does not contain thiosermal (mercury).
A lot of parents are (probably unnecessarily) worried about thiomersal in vaccines. The reassurance from NHS would probably be more persuasive if they knew how to spell it. (Thiomersal is, confusingly, called “thimerosal” in the US, but not “thiosermal”.)