Unvaccinated young adults drive measles outbreak in Mongolia

A 2015 measles outbreak in Mongolia had a particularly high incidence among young adults born outside of the capital city of Ulaanbaatar, according to findings published in The Journal of Infectious Diseases.

Researchers reported that nosocomial transmission worsened the outbreak.

“The National Verification Committee for measles elimination provided documentation that endemic measles virus transmission had been interrupted for a period [of more than] 36 months, and the achievement of measles elimination in Mongolia was verified by the [Western Pacific Region] Verification Commission in March 2014.” Jose E. Hagan, MD, MS, of the global immunization division of the CDC, and colleagues wrote. “However, after verification of elimination, a large nationwide measles outbreak occurred in March 2015, continuing until July 2016 in two distinct waves with different epidemiologic characteristics.”

Hagan and colleagues investigated the outbreak, which included performing a matched case-control study of young adults with laboratory-confirmed measles in Ulaanbaatar. The researchers matched patients with measles aged 15 to 28 years with two to three controls living in the same neighborhood and used conditional logistic regression to estimate adjusted matched odds ratios for risk factors.

From March 1, 2015, to Sept. 30, 2015, 20,077 suspected cases of measles were reported, the researchers wrote. Of these, 14,010 were confirmed.

Being unvaccinated was a risk factor for measles infection (adjusted matched OR = 2; P < .01), as was being a high school graduate with no college education (OR = 2.6; P < .01). Other risk factors included remaining in the city during the outbreak (OR = 2.5; P < .01), exposure to an inpatient health care center (OR = 4.5; P < .01) and being born outside of the city (OR = 1.8; P = .02).

In an accompanying editorial, David N. Durrheim, MBChB, DrPH, DCH, of the School of Medicine and Public Health at the University of Newcastle, Australia, pointed out that although the outbreak occurred after a period of high vaccination coverage in Mongolia, a significant portion of patients were born during the dissolution of the Soviet Union, and they had no history of vaccination.

“The 2015 to 2016 Mongolian outbreak shows graphically the threat that sizeable undetected immunity gaps pose to measles elimination,” Durrheim wrote. “Its dramatic spread and scale demonstrate the unique transmission potential and force of infection of the measles virus in inadequately protected populations. The global public health community ignores the stark lessons from this outbreak at our peril.” – by Andy Polhamus

Disclosures: The authors report no relevant financial disclosures.