Moderna has reported that its COVID-19 vaccine has been found to be 96% effective among 12 to 17 year olds in its first clinical trials. 

Among the 3,235 participants in trials in the United States, two-thirds of the subjects were given the vaccine and one-third received a placebo. The vaccine efficacy against COVID-19 was shown to be 96%. The company stated that mRNA-1273 was well tolerated with no serious safety concerns detected. There were 12 cases of COVID-19, 14 days after the first dose. 

For these intermediate results, participants were followed-up for on average 35 days following the second dose… (NDTV - Agence France-Presse, May 6, 2021)