Until measles vaccine (MV) Schwartz vaccine became compulsory in 1982, measles outbreaks occurred every other year. Thereafter the annual incidence per 100,000 of measles sharply dropped from 213 in 1982 to 57 in 1991, when the Schwartz vaccine was substituted by Edmonston Zagreb, major outbreaks of measles then occurred 4-5 years apart. A two-fold increase in the annual incidence of measles in Riyadh City from 7 per 100,000 children in 1995 to 14 per 100,000 in 1996, principally affecting students in public schools, prompted us to study the potential risk factors for transmission of measles in Riyadh City.
Methodology
We established a special surveillance system for daily reporting of clinically diagnosed measles cases throughout the outbreak period October 1996 to June 1997. We compared potential risk factors for measles in 131 case-students identified in the midst of the outbreak, to 223 control-students, matched by school and class. For estimating odds ratio [OR] for measles vaccination unvaccinated students were used as reference category.
Results
From October 1996 to June 1997, 482 cases of measles were reported from 103 schools all over Riyadh; the male to female ratio was 2:1. Male children aged 10-14 years had the highest age-specific attack rate (ASAR) 49/100,000, while the ASAR for females in this age group was 20/100,000. Thirty (23%) out of the 131 cases and 29 (13%) out of the 223 controls were never vaccinated against measles (OR 2.0, 95% confidence interval [95% CI] of 1.1-3.6). School children who received Measles-Mumps-Rubella vaccine (MMR) 10 years or more prior to the outbreak (OR = 0.5, 95% CI 0.1-1.6) were more likely to contract measles in contrast to those who were vaccinated within the last nine years (OR = 0.3, 95% CI 0.1-0.6). Other risk factors for contracting measles included attending any health facility prior 2 weeks of onset, 52% of the cases and 19% of the controls (OR = 4.6, 95% CI 2.9-7.5), or exposure to a friend in the same school with measles, 65% and 30% for cases and controls respectively (OR = 4.3, 95% CI 2.6-7.2).
Conclusion
As mostly older children and more males were affected during the latest outbreak, data suggested waning immunity and there was no apparent protective effect for MV (Schwartz vaccine) given 10 years ago. A booster dose of MMR for male adolescents is recommended as MMR routinely given to females in high schools explains their low ASAR.