Large Measles Outbreak in a High School in Canada: Lower than Expected Vaccine Efficacy in Two-dose Recipients and Higher Risk With Younger Age at First Dose

Background: Seven of the ten Canadian provinces recommend two doses of measles vaccine at 12 and 18 months of age. This is expected to provide a ≥99% protection. Between January and August 2011, >700 measles cases were reported in the province of Quebec, Canada. A high school was especially affected with 98 cases. We report the preliminary results of the school outbreak investigation which showed lower than expected vaccine efficacy (VE) with the recommended schedule.

Methods: Cases meeting the criteria of the national measles case definition were identified by passive followed by active surveillance. Vaccination status was ascertained from written vaccination records. VE was calculated as (1- RR vaccinated/unvaccinated) x 100%

Results: Among the 1306 students, 4.6% were unvaccinated. Among the 98 measles cases identified, 50% were unvaccinated and 42% had received two doses. The overall attack rate (AR) was 7.5%, varying from 9.3% in grade 7 to 9 to 4.7% in the older grades.

Number of vaccine doses	0	1	≥2	Vaccinated no written proof	Unknown	Total
Cases n (Attack rate)	49 (81.7%)	3 (3.4%)	41 (3.7%)	2 (7.1%)	3 (23.1%)	98 (7.5%)
Non Cases n	11	86	1075	26	10	1208
Total n (% of total)	60 (4.6%)	89 (6.8%)	1116 (85.5%)	28 (2.1%)	13 (1%)	1306

VE for two doses was 95.5% (95% CI: 93.8%-96.7%) In the two-dose recipients, the risk of measles varied with age at first dose: the attack rate was 4 times higher in those who received their first dose between 12 to 14 months of age compared to 15 months and older (4.3% versus 1.0%, RR=4.1). Attack rates were not affected by the interval between the two doses.

Conclusions: In this large school outbreak, nearly half the cases were two-dose recipients and VE was lower than expected. Despite a second dose administered 6 months after the first dose given at 12-14 months, the risk of measles is significantly higher than with a first dose given at ≥15 months.  If these results are confirmed in other settings, the recommended age at first dose will have to be reconsidered.