Measles prevention in Saudi Arabia includes compulsory measles vaccine (MV) before 1 year of age, optional MMR after 1 year and proof of vaccination for school entry. In March 1994, we learned of 5 measles cases confirmed by anti-measles IgM in students of the Indian Embassy School (IES) in Riyadh. We began an investigation to determine the reason for measles transmission under this policy.
Methodology
We defined an outbreak-associated case of measles as a febrile (≥ 38.3o C) illness with a generalized maculopapular rash ≥ three days and cough, coryza or conjunctivitis occurring between February and April 1994 at IES. For each case, we selected at random from the same class 5 control-students without current or a past history of measles. We interviewed parents of both case- and control-students about the students' history of MV. From these histories we used the formula (1 - odds ratio) to compute vaccine effectiveness (VE), using unvaccinated status as a reference.
Results
For three months after the onset of a rash in an unvaccinated 9-year-old Indian visitor to Riyadh, 60 IES students (attack rate 11/1000) developed measles. Seven of the case-students were unvaccinated, compared with 29 of 294 controls (VE=17%; 95% confidence interval [CI] -58%, 67%). Among vaccinated children the mean age for primary MV was lower for case-students than control-students both with and without documentation of MV (p<0.05, Kruskal-Wallis test). 95% confidence intervals of VE estimates included 90% VE only for documented MV given after 9 months of age (VE=82%; 95% CI -38%, 99%). MV from 6 to 9 months was not effective either with (VE=2.2%; 95% CI -34%, 34%) or without (VE=-15%; 95% CI -70%, 58%) later MMR.
Conclusion
The investigation showed the importance of ensuring documented measles vaccination for school enrollment as required by Ministry of Health regulations. However, the reduced effectiveness of MV given before 10 months of age suggest that maternally acquired immunity is contributing to MV failure under the present policy. We are also concerned with the failure of MMR to boost these early measles vaccinations.