Once a safe, effective vaccine already exists, giving some participants in a vaccine study a placebo means knowingly withholding protection, write Drs. Richard Rupp and Megan Berman. So, when scientists need to test an updated version of an existing vaccine, instead of giving half the group a placebo, scientists compare the immune response from the new version to what was seen in earlier, already successful trials, Berman and Rupp explain in their Vaccine Smarts column.