Nation Moderna says its COVID vaccine found to be 100% effective in children 12 to 17 two weeks after second dose May 25, 2021Elizabeth Weise and Karen Weintraub, USA TODAYComments Off on Moderna says its COVID vaccine found to be 100% effective in children 12 to 17 two weeks after second dose The Moderna vaccine was 93% effective against COVID-19 in teenagers aged 12 to 17 after the first dose and 100% two weeks after the second dose.