News Moderna says its COVID-19 vaccine found to be 100% effective in children 12 to 17 two weeks after second dose May 25, 2021May 25, 2021Elizabeth Weise and Karen Weintraub, USA TODAYComments Off on Moderna says its COVID-19 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.