title: Two-year prospective study of relative risk of a second cerebral concussion.
abstract: OBJECTIVE
  To prospectively measure the relative risk of cerebral concussion among those with a history of concussion compared with those having no previous concussions by using a population of high school and college football players.
  
  
  METHODS
  A representative national sample of high school and college football players was followed for two football seasons over a 2-yr period (1997-1998) as part of a national football injury surveillance project. There were a total of 15,304 player-seasons and over 1 million athlete-exposures to the possibility of injury in practices and games; 975 of the player-seasons (6.4%) had a history of concussion in the previous 5 yr.
  
  
  RESULTS
  There were 572 concussions recorded, 161 among those with a history (16.5%) and 411 among those with no history (2.9%). Relative risk for individuals with a history of concussion is 5.8 times greater than for individuals with no history (95% confidence interval, 4.8-6.8).
  
  
  CONCLUSIONS
  This large prospective cohort study indicates the risk of sustaining a cerebral concussion is nearly six times greater for individuals with a history of concussion than for individuals with no such history.
authors.affiliation: Exercise Research Associates, Ann Arbor, Michigan, USA.
authors.forename: Eric D
authors.initials: ED
authors.lastname: Zemper
mesh_terms: D000293:Adolescent; D000328:Adult; D001265:Athletic Injuries / Q000453:epidemiology*; D001924:Brain Concussion / Q000145:classification / Q000453:epidemiology*; D001930:Brain Injuries / Q000453:epidemiology*; D015331:Cohort Studies; D003627:Data Interpretation, Statistical; D005538:Football / Q000293:injuries*; D006801:Humans; D011159:Population Surveillance; D011446:Prospective Studies; D012008:Recurrence; D018570:Risk Assessment; D012494:Sampling Studies; D014481:United States / Q000453:epidemiology