Children, the elderly, pregnant women, and other different populations may need different medication and vaccine doses to optimize safety and efficacy. But using conventional clinical trials to determine dosing for different populations can be difficult for ethical and logistical reasons. Virtual patients that vary physiologically can be used to understand how new vaccines and medicines work in different people. Using virtual patients, virtual trials can test vaccines, medicines, and combinations of medicines at different doses to aid dose selection before conducting clinical trials. For 20 years, Certara has used virtual patients to accelerate bringing medicines to all patients.