Early Action is a non-binding early program, meaning that if you are admitted you are not obligated to enroll. If you apply to Harvard under our Early Action program, you may also apply at the same time to any public college/university or to foreign universities but you are restricted from applying to other private universities’ Early Action and Early Decision programs.
https://college.harvard.edu/admissions/apply/application-timeline/restrictive-early-action
Other top 20 private EA are same to Harvard, except MIT. Check the school website for sure.