Two-thirds of US teenagers and young adults ages 15 to 24 have had oral sex, according to researchers, who say people in this group may mistakenly feel it is less risky than vaginal intercourse.

This is the first time researchers asked young people about the timing of oral sex relative to vaginal sex, according to Thursday’s findings by the US Centers for Disease Control and Prevention.

It is important to understand sexual activities of young people to help educate them about risks, the report’s authors wrote.

The study, based on 6,346 interviews that followed behavior from 2007 through 2010, determined that 66 percent of females and 65 percent of males had experienced oral sex. About 25 percent of both genders had oral sex at least once before they had vaginal intercourse, the survey found.

‘‘Research suggests that adolescents perceive fewer health- related risks for oral sex compared with vaginal intercourse,’’ wrote the authors, led by Casey Copen in the division of vital statistics for the Atlanta-based CDC.