Mumps outbreak in a highly vaccinated population, Al-Faisaliyah district, Riyadh, March-June, 1998

Saudi Arabia started requiring mumps, measles, rubella (MMR) vaccinations for preschool populations in 1991. Since the introduction of the mandatory vaccination of MMR to the population, the result has been a substantial decrease in reported cases of mumps. From 1991 to 1997, national coverage of infants with the recommended schedule has ranged between 76% and 91%. However, in 1998 an outbreak of mumps occurred in the district of Al-Faisaliyah. This outbreak affected adults as well as vaccinated and unvaccinated children. An epidemiological investigation was initiated to determine the causes of the outbreak and vaccine effectiveness.

We defined an outbreak-associated case of mumps as a case of acute onset of swelling obscuring the angle of the jaw for at least 2 days in a resident of Al-Faisaliyah district from March to June 1998. A retrospective family cohort study was conducted among families with mumps cases. We obtained written vaccination histories of patients and family members, ages 1 to 15, living in the same quarters. Subjects without documented vaccination histories were excluded. Attack rates (AR) and Vaccine Effectiveness (VE) were computed. The VE was computed by comparing mumps AR for different categories of vaccinated children to the unvaccinated reference category. Sixty-four cases, which met the definition of confirmed mumps cases, were reported. This represents an AR of 6.0 per 1000 persons. The first reported case of mumps occurred in a 12-year-old child. A range of 2 to 8 mumps cases occurred per week between March 1, 1998 and June 30, 1998. The epidemic peaked between May and June with 6 to 8 cases reported weekly (Figure 1). No additional cases of mumps prior to the first reported case were identified through retrospective record review.

The median age of mumps cases was 12 years (range: 9 months to 33 years). Of the 64 cases, children 10 to 14 years of age had the highest incidence rate of mumps with an AR of 31 per 1000 population. A family study was carried out and 230 children (including 46 mumps cases) were involved. Ninety-one percent of these children did have the mumps containing vaccine (MCV). Twenty-one children (out of 230) did not receive an MCV (9%) and 7 developed mumps for an AR of 33%. In contrast, 39 of 209 children who were vaccinated developed mumps for an AR of 19% and a VE of 44% (95% confidence interval [CI] -9, 71). In addition, among 76 children who received MCV during the 4 years previous to the outbreak, 7 developed mumps for an AR of 9% and VE of 72% (95% CI 30, 89). Out of 133 children vaccinated more than 4 years before, the AR was 24% and the VE was 28% (95% CI -42, 63). Age at vaccination, type of vaccine, and place of vaccination were not found to be a risk factor for contracting mumps disease.

Mumps outbreaks in highly vaccinated populations have been noted by several reports; the occurrence of these outbreaks has established that mumps vaccine-failure plays a role in the continued mumps outbreaks in those populations [1, 2. 3]. The MCV should produce a protective antibody in over 95% of vaccinated persons and protection should last for 15 years [4]. However, this mumps outbreak propagated in a highly vaccinated population in primary and intermediate schools. During this outbreak, our results indicated that the vaccine gave protection to 44% of the vaccinated group and maximum protection to the children who received MCV during the previous 4 years. The attack rate of mumps among females was lower than that among males. Three factors could probably explain this: the MMR vaccine is given to girls at age 12 for rubella control, the girls attended an intermediate school in a nearby district, and the girls in the primary school were exposed 9 weeks after the boys' schools. Accordingly, both primary and secondary MCV failure allowed this outbreak to occur. We recommend a serosurvey for antibody against mumps in representative areas unaffected by the recent mumps outbreak. Training of medical personnel at both the district and the sector level is essential in order to detect and compact further outbreaks.