Measles outbreak Riyadh city, 1997

In December 1996, there was a twofold increase in the number of measles cases among school age children in Riyadh City as compared to the number of cases that occurred in 1995. The School Health Units (SHU) vaccinated students with MMR vaccine. The Field Epidemiology Training Program was asked 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 relatives, friends and neighbors of cases. The vaccination status of 131 cases and 223 controls, matched by school and class were ascertained. Information about number of cases, total number of students, and date of school vaccination were collected from 44 schools.

From October 1996 to June 1997, 482 cases of clinically-diagnosed measles were reported from 103 schools all over Riyadh city. The mean age in years of cases was 13.3 for males, and 11.8 for females; the inter-quartile range for age was 9-16 years. The age-sex specific attack rate (ASAR) for measles was highest among male children aged 10-14 years, and lowest in children aged 1-4 years (Table). Thirty (23%) out of 131 cases and 29 (13%) out of 223 controls were never vaccinated against measles (OR=2.0, 95% CI= 1.1-3.6). Sixty-one cases (47%) and 131 controls (58.7%) received a single vaccination against measles (MV or MMR) whereas 39 cases (30%) and 63 controls (28.3%) received the two routinely scheduled vaccinations against measles. 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), visiting a hospital outpatient department (OR=4.3, 95% CI=2.3-8.0), 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) and visiting a person with measles within two weeks prior to onset of symptoms (OR=3.1, 95% CI =1.1-9.2).

SHU were able to vaccinate all student in 14 (31.8%) schools, a variable number of students in six (13.6%) schools, and no students in 24 (54.6%). The vaccination campaigns began a median of 14 days after the date of diagnosis of the first case of measles in the school ( range 0-56 days). In fully vaccinated schools vaccination within 10 days after the onset of the first case and yielded a PF% of 59.5% (95%CI 40.6-72.4) compared with 2.1% (95% CI=0.0-32.7) for a delay of 19 days or more (Figure).

Measles virus is highly infectious and can result in outbreaks in schools and other places where susceptible contacts convene. An infected person was believed to have transmitted measles to dozens of others at a sport meeting[1]. In this outbreak 50% of measles cases acquired their infection in a medical setting. In urban areas, visits to hospitals; including emergency rooms and immunization clinics, contributed in perpetuating measles transmission during epidemics; some studies confirmed that 3-20% of measles cases acquired their infection in medical settings[1,2). Exposure to measles virus in medical settings could have occurred while children were waiting to see a doctor for some other health problems. Airborne transmission via aerosolized droplet nuclei has been documented in closed areas e.g. a doctor's examination room, two hours after a person with measles occupied the area[3]. Prevention of measles transmission in the medical facilities must include screening of children with rash at waiting areas so that patients with measles can be isolated promptly.

To contain outbreaks in schools, the vaccination campaign should be started within the first incubation period. A study in the USA, confirmed that vaccination of the schools during the first incubation period will stop inter-school transmission of measles[4]. Efforts should be made to immediately vaccinate all students in areas where an unusual increase in the number of measles cases is noted.

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, and is probably due to girls' exposure to a routine booster dose of MMR at the secondary schools.

Two factors would probably explain the low efficacy of MV: the strain of the vaccine and the age of its administration. The VE of the MV (Schwartz vaccine) given at age of nine before 1991, was as twice as that of MV (EZ vaccine) administered at the age of six months (56% vs. 26%).