1 / 11The world’s biggest Covid-19 vaccine study got underway Monday with the first of 30,000 planned volunteers helping to test shots created by the U.S. government -- one of several candidates in the final stretch of the global vaccine race.2 / 11There’s still no guarantee that the experimental vaccine, developed by the National Institutes of Health and Moderna Inc., will really protect.3 / 11The needed proof: Volunteers won’t know if they’re getting the real shot or a dummy version. After two doses, scientists will closely track which group experiences more infections as they go about their daily routines, especially in areas where the virus still is spreading unchecked.4 / 11“Unfortunately for the United States of America, we have plenty of infections right now” to get that answer, NIH’s Dr. Anthony Fauci recently told The Associated Press.5 / 11Moderna said the vaccination was done in Savannah, Georgia, the first site to get underway among more than seven dozen trial sites scattered around the country.6 / 11In Binghamton, New York, nurse Melissa Harting said she volunteered as a way “to do my part to help out.”“I’m excited,” Harting said before she received a study injection Monday morning. Especially with family members in front-line jobs that could expose them to the virus, “doing our part to eradicate it is very important to me.”7 / 11Several other vaccines made by China and by Britain’s Oxford University began smaller final-stage tests in Brazil and other hard-hit countries earlier this month.8 / 11But the U.S. requires its own tests of any vaccine that might be used in the country and has set a high bar: Every month through fall, the government-funded Covid-19 Prevention Network will roll out a new study of a leading candidate -- each one with 30,000 newly recruited volunteers.9 / 11The massive studies aren’t just to test if the shots work — they’re needed to check each potential vaccine’s safety. And following the same study rules will let scientists eventually compare all the shots.10 / 11Next up in August, the final U.S. study of the Oxford shot begins, followed by plans to test a candidate from Johnson & Johnson in September and Novavax in October -- if all goes according to schedule. Pfizer Inc. plans its own 30,000-person study this summer.11 / 11That’s a stunning number of people needed to roll up their sleeves for science. But in recent weeks, more than 150,000 Americans filled out an online registry signaling interest, said Dr. Larry Corey, a virologist with the Fred Hutchinson Cancer Research Institute in Seattle, who helps oversee the study sites.