From March through June, 1995, a measles outbreak affected the town of Haqel, Saudi Arabia (population 19000) and nearby villages. Since measles vaccination in Saudi Arabia is required at age 6 months, at one year, and again before school entry, we started an investigation to determine the reason for measles transmission under this policy.
Methodology
We defined an outbreak-associated case of measles as a rash illness between March and June 1995 in Haqel districts with fever ≥ 38.5oC and one or more of the following: conjunctivitis, coryza or cough. We assessed coverage of measles containing vaccine (MCV) with a single stage cluster survey of 134 randomly selected Haqel households. To evaluate measles vaccine effectiveness, we compared documented measles vaccination of measles cases to the surveyed children using unvaccinated status as referent group.
Results
From March though June, 1995, 194 cases of measles (attack rate [AR] = 8.3 per 1000) occurred in Haqel. School children (AR=28 per 1000) were more affected than other groups. The vaccination survey involved 473 children under 14 years of age, including 228 preschool children, and 245 school children. Coverage with documented MCV in preschool children was 89% [95% CI=83-93] for one or more doses and 71.7% [95% CI=65-78] by two or more doses of MCV. In school children, coverage was 51% [95% CI 41-60] for one dose of MCV and 31% [95% CI=22-40] for two doses or more MCV. Vaccine effectiveness for one dose of MCV given between six and nine months of age was 77% [95% CI=60-87] for all children, 75% [95% CI=18-94] for preschool children, and 80% [95% CI=67-93] for school children. For both groups effectiveness of two or more documented doses of MCV was 100%.
Conclusion
The outbreak was associated with under vaccination and with decreased effectiveness of a single dose of MCV given between six and nine months of age. Compliance with Ministry of Health policy should minimize the possibility of future measles outbreaks among school and preschool populations in Saudi Arabia.