Visiting health centers as a risk factor for measles transmission: Kamwenge District, Uganda, April – July 2015

Abstract

Uganda has been implementing a one-dose measles vaccination at age 9 months in its national EPI schedule. On 27 April 2015, a measles outbreak, which was confirmed by serum positivity in several patients, occurred in Kamwenge District. Since then, the number of reported measles patients has increased despite the implementation of measures to control the outbreak by the local government. We investigated this outbreak to identify the risk factors for measles transmission, estimate vaccination coverage, determine vaccine effectiveness, and recommend control measures. We defined a probable case as onset in a Kamwenge District resident of fever and generalized rash from 16 April 2015 onward with ≥ 1 of the following: coryza, conjunctivitis, or cough. A confirmed case was a probable case with positive measles-specific IgM in patient serum. For case-finding we reviewed medical records and found patients in the community with the help of the village health team. We determined vaccination histories by vaccination cards or interviews. In a case-control study, we compared the exposure histories of 50 probable case-persons with 200 asymptomatic control-persons during case-persons’ exposure period (i.e., between minimum and maximum incubation). We matched case- and control-persons by age and residence village. We estimated vaccination coverage for children aged ≤ 2 years based on the percent of control-children vaccinated. We identified 213 probable/confirmed cases from 3 affected sub-counties (attack rate = 5.1/10,000). The epidemic curve showed sustained community transmission. The case-control study showed that 42% (21/50) of case-persons and 12% (23/200) of control-persons visited health centers during case-persons’ exposure period (AORM-H = 6.1; 95% CI = 2.7-14). Vaccination coverage among children aged ≤2 years was 58% (95% CI = 47-68%). The vaccine effectiveness was 80% (95% CI=35-94%). We found that all health centers were crowded, with no triaging system to separate suspect measles patients from patients with other illnesses. Exposures to measles patients at crowded health centers, low vaccination coverage, and suboptimal vaccine effectiveness facilitated measles transmission in this outbreak. We recommended an emergency immunization campaign targeting young children, triaging and isolating suspect measles patients at health centers, and introducing a second dose of measles vaccine in the immunization schedule.