Risk factors for measles and vaccine efficacy during an epidemic in Montreal

1) to identify potential risk factors for measles and 2) to assess the efficacy of the measles vaccine.

Cases of measles reported from primary and secondary schools in Montreal during the 1989 epidemic were compared with classroom and sibling controls with respect to age at vaccination against measles, time since vaccination, type of vaccination, type of providers and age of the mother.

Subjects vaccinated at 12 months had an OR of 3.9 (CI 95%: 2.1 to 7.0) of contracting measles when compared with subjects vaccinated at 15 months or later. Vaccination at age 13 or 14 months was not associated with an increased risk. Subjects vaccinated before June 1979 had an OR of 5.8 (CI 95%: 2.8 to 12.1) of developing measles compared with subjects vaccinated after June 1980. Vaccine efficacy was 96.1%.

In highly vaccinated populations, vaccination at 12 months and vaccination before 1980 are probably two reasons why outbreaks still occur. The two-dose schedule could reduce the proportion of vaccinated persons who remain susceptible.