The world's largest COVID-19 vaccine study is starting today.
About 30,000 total volunteers will help test the shots created by the National Institutes of Health and Moderna.
The study will see if the experimental vaccine they developed really works in a real world setting and not just a lab.
Volunteers won't know if they're getting the real shot or a placebo, and after two doses, scientists will then track which group experiences more infections as they go about their everyday routines.
Several other vaccines made by China and Britain's Oxford University have also started some final-stage testing in other countries.