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Outbreak of Measles among School Students: Risk Factors for Transmission and Evaluation of Two Vaccination Strategies, Riyadh, November 24, 1996 - June 1, 1997.


In December 1996, there was a two-fold increase in the number of cases of measles among children at school age in Riyadh City as compared to the number of cases that occurred in 1995. The School Health Units (SHU) vaccinated students with MMR vaccine; however, not all students in all schools were vaccinated. The objectives of this study were to identify potential risk factors for transmission of measles, and to evaluate the control measures instituted during the outbreak


Surveillance of clinically diagnosed measles was activated to identify additional cases of measles in primary health care centers (PHCCs), schools, SHU, governmental and private hospitals, and among the relatives, friends and neighbors of cases. The role of some potential risk factors, including the vaccination status of 131 cases and 223 controls, matched by school and class were ascertained.


From October 1996 to June 1997, 482 cases of clinically-diagnosed measles were reported from 103 schools all over Riyadh, 5 (9) days after their diagnosis. Cases occurred in all age groups. The mean age in years (SD) of cases was 13.3(6 for males, and 11.8 (9 for females; the inter-quartile range for age was 9-16 years. Only 7% of cases occurred among children aged <5 years. The age-specific attack rate (ASAR) for measles was highest among children aged 10 -14 years (35/100,000 children). The male:female sex ratio was 2:1. Thirty (23%) out of 131 cases and 29 (13%) out of 223 controls were never vaccinated against measles (OR 1.99, 95% CI 1.09-3.63). Sixty-one cases (47%) and 131 controls (58.7%) received a single vaccination shot against measles (MV or MMR), whereas 39 cases (30%) and 63 controls (28.3%) received the two routinely scheduled vaccinations against measles. The attributable risk fraction percent (AF%) was consistently smaller when the SHU vaccinated all students at schools compared with vaccination of selected target groups within schools. A delay in vaccination of schools for 10 days resulted in a 40% increase in the AF%, and a 180% increase by the end of the third week. Similarly, the relative risk (RR) doubled by the end of the second incubation period. School children who received MMR 10 years or more prior to the outbreak were more likely to contract measles as compared with those who were vaccinated within the last nine years (OR=0.48, 95% CI 0.13-1.63). Other risk factors for contracting measles included being at a health facility, a PHCC (OR=3.95, 95% CI 2.2-6.9), or a SHU (OR=6.1, 95% CI 3.3-11.4), OPD (OR=4.3, 95% CI 2.3-8.0), or exposure to a friend in the same school with measles (OR=4.3, 95% CI 2.6-7.2), having a relative with measles (OR =3.7, 95% CI 1.2-7.5), or being visited by a case of measles within 2 weeks prior the onset of symptoms (OR =3.1, 95% CI 1.1-9.2).


The periodicity of outbreaks of measles observed in the surveillance data of the Riyadh region showed that this outbreak was predictable. Since 1983, major outbreaks had been occurring in cycles, once every five years. During this outbreak, the incidence of measles among girls was lower than that of boys. The lower age-sex specific attack rate of measles among girls sharply declined after the age of 14 years, which is probably due to girls' exposure to a routine booster dose of MMR at the secondary schools. This study showed that medical facilities, schools and households played a momentous role for measles transmission during this outbreak. Moreover, the study showed that delay in the institution of the vaccination campaign after the first incubation period resulted in a two-fold increase in the RR and the AF% for contracting measles. During this outbreak, unvaccinated teenagers or children who received MMR more than 10 years prior to this outbreak, were more likely to contract measles; and there was no apparent protective effect for MV (Schwartz vaccine) given 10 years ago. The results of this study showed that the efficacy of the MMR (Schwartz vaccine), currently used in Saudi Arabia, was moderately high for at least 10 years. On the other hand, the efficacy of MV (Schwartz vaccine) was relatively lower and unsatisfactory. Two factors may probably explain the low efficacy of MV vaccines: the strain of the vaccine and the age of its administration.