The FDA's Center for Biologics Evaluation and Research is the government agency responsible for regulating vaccines in the United States. Working in conjunction with the CDC and the NIH, they continuously research and monitor vaccine safety and effectiveness. New vaccines are licensed only after thorough laboratory studies and clinical trials, and safety monitoring continues even after a vaccine has been approved. There have been — and will continue to be — improvements (such as those that have already been made to the DTaP and polio vaccines, for example) that will minimize potential side effects and ensure the best possible safety standards.