Serosurvey of measles, mumps and rubella antibodies in Saudi children.

OBJECTIVE: A serosurvey study to evaluate the proportion of children with antibodies against diseases targeted by the Expanded Program of Immunization in the Kingdom of Saudi Arabia.

METHODS: Using multistage sampling techniques, we collected samples and sent them for laboratory assay from the following age groups; 100 samples at 6 months, 12 months, 18 months, 6 years, 13 years, and 17 years. We conducted the study from September 2001 to February 2002. We assayed sera for measles, rubella, and mumps antibodies in the measles-mumps-rubella reference laboratory in Germany, using enzyme immunoassay and plaque neutralization (PN) as a backup test for equivocal and negative samples. We only carried out a backup test for measles samples.

RESULTS: The age group of 6 months had the highest proportion with negative measles antibodies. After adding the backup test (PN), the proportions of children with protective measles antibody were; 64% at 6 months, 87% at 12 months, 91% at 18 months, 75% at 6 years, 96% at 13 years, and 98% at 17 years. Rubella antibody positivity rates (>7 IU) were 28% at 6 months, 49% at 12 months, 97% at 18 months, 98% at 6 years, and 100% at 13 years. While positivity rates in mumps were 14% at 6 months, 29% at 12 months, 59% at 18 months, 64% at 6 years, and 75% at 13 years.

CONCLUSION: The unexpected low proportion of children with protective level at 6 years, despite being vaccinated with 2 measle doses is an important phenomenon. This reflects the interference between the first and the second measles dose. The Ministry of Health decided to conduct a catch up campaign targeting 1st through 3rd grade primary schools, who did not catch the mass campaign conducted in 2000. Also, this supports the decision taken by the ministry to change the measles immunization schedule to MMR at 12 months and a second dose at 6 years of age.