nprnews_____ = ||| lubuntu at wor3|||How Can You Tell If A COVID-19 Vaccine Is Working?||| kickerpage = |||no kicker pagenpr for NPRNEWS||| summarynpr = |||Several COVID-19 vaccine candidates are being tested now. But why does it take 30, 000 volunteers to know if one is safe and effective? And what does it mean to say a vaccine candidate is working?