Abstract
In an outbreak of measles in North Wales centred on a secondary school in 1991, 74 cases occurred over a period of 51 days. Before the outbreak started, 27% pupils did not have a history of measles or immunization and were considered susceptible. Active case finding and identification and vaccination of susceptible contacts commenced after the fourth generation of cases and further reduced the pool of susceptible schoolchildren from 17%, at the onset of the vaccination campaign, to 8%. A fifth generation of cases did not occur. Delays in diagnosis (mean 2.8 days) and notification (mean 6.1 days) hampered control. There was no evidence of primary vaccine failure (observed vaccine efficacy 97%). Sixty-nine (93%) cases were considered preventable. Reasons for the apparent success of the intervention are discussed.