Advertisement Pfizer said Monday that if it makes changes to its vaccine testing in children, it will provide an update then. The New York-based company is testing its vaccine in as many as 4,500 children in the United States and Europe. The FDA said in a statement it could not comment on its discussions with companies, but added, “we do generally work with sponsors to ensure the number of participants in clinical trials are of adequate size to detect safety signals.” The news was first reported by the New York Times. U.S. officials and independent medical experts said last month the benefits of the vaccines far outweigh the risks of the side effect, which has been reported in several hundred people younger than 30. But any vaccine safety issues could slow uptake, particularly among parents wary of taking any health risk with their children.