Risk Factors for Measles Virus Infection Among Adults During a Large Outbreak in Postelimination Era in Mongolia, 2015

Abstract

Background: In 2015, a large nationwide measles outbreak occurred in Mongolia, with very high incidence in the capital city of Ulaanbaatar and among young adults.

Methods: We conducted an outbreak investigation including a matched case-control study of risk factors for laboratory-confirmed measles among young adults living in Ulaanbaatar. Young adults with laboratory-confirmed measles, living in the capital city of Ulaanbaatar, were matched with 2-3 neighborhood controls. Conditional logistic regression was used to estimate adjusted matched odds ratios (aMORs) for risk factors, with 95% confidence intervals.

Results: During March 1-September 30, 2015, 20 077 suspected measles cases were reported; 14 010 cases were confirmed. Independent risk factors for measles included being unvaccinated (adjusted matched odds ratio [aMOR] 2.0, P < .01), being a high school graduate without college education (aMOR 2.6, P < .01), remaining in Ulaanbaatar during the outbreak (aMOR 2.5, P < .01), exposure to an inpatient healthcare facility (aMOR 4.5 P < .01), and being born outside of Ulaanbaatar (aMOR 1.8, P = .02).

Conclusions: This large, nationwide outbreak shortly after verification of elimination had high incidence among young adults, particularly those born outside the national capital. In addition, findings indicated that nosocomial transmission within health facilities helped amplify the outbreak.

Keywords: case-control; disease outbreak; measles; nosocomial infection.

Figure 1.

Clinically confirmed (gray, n = 13 064) and laboratory-confirmed (black, n = 946) measles cases by week of rash onset—Mongolia, March 1–September 30, 2015 (N = 14 010). Note: During May 15–June 5, 2015 (indicated by dotted lines), a nationwide outbreak response immunization (ORI) campaign was implemented using measles vaccine with a target age group of 6–71 months, 371 971 doses were delivered, and the ORI administrative coverage was 94%.

Figure 2.

(A) Age-specific proportion of confirmed measles cases by epidemiological week, and (B) histogram of confirmed cases (N = 14 010) by epidemiological week—Mongolia, March 1–September 30, 2015. Note: During May 15–June 5, 2015, a nationwide outbreak response immunization (ORI) campaign was implemented using measles-containing vaccine with a target age group of 6–71 months, 371 971 doses were delivered, and the ORI administrative coverage was 94%.

Figure 3.

Vaccination coverage (right axis) of first dose (measles-containing vaccine [MCV]1, solid line) and second dose (MCV2, dashed line) of measles-containing vaccine (World Health Organization-UNICEF estimates [16]), supplemental immunization activities ([SIA] dotted lines), and MCV1 by 1 year of age according to Multiple Indicator Cluster Survey [–21] (diamonds; details of each SIA are provided in the inset table); and age-specific attack rate (black bars, left axis) of confirmed measles (N = 14 010 cases) during the outbreak—Mongolia, March 1–September 30, 2015. Note: MCV2 was introduced in 1985, but coverage estimates were available only from 2000.

Figure 4.

Scatterplot of confirmed measles cases that reported exposure to a healthcare facility 1–60 days before rash onset—Mongolia, March 1–September 30, 2015 (N = 496). Each filled circle represents 1 case with healthcare exposure that occurred 7–21 days before rash (measles incubation period) onset (67% of total). Open circles represent cases with healthcare exposure 1–6 or 22–60 days before rash onset (33% of total).