Much of the world is watching and waiting for researchers to develop a vaccine for the disease COVID-19. But making enough of the vaccine to end the coronavirus health crisis will be the biggest medical manufacturing feat in history. That work has started. Researchers are currently setting up testing, involving 10,000 to 30,000 volunteers for every candidate vaccine. These scientists say they hope to get an answer on whether a vaccine works by as early as this October. However, health officials face a hard reality. The developer of an effective vaccine may not be able to make and deliver billions of doses all at once.