nprnews_____ = ||| lubuntu at wor3|||Why Tens Of Thousands Of People Are Key To Testing A COVID-19 Vaccine||| kickerpage = |||no kicker pagenpr for NPRNEWS||| summarynpr = |||Volunteers getting the shot help determine if a candidate vaccine works. But what with social distancing and masks, scientists must discern if it’s the shot or these other measures preventing illness.