'Susceptibility Pockets' Driving Uptick in US Measles Cases

Bridget M. Kuehn

October 03, 2017

An uptick in United States measles incidence between 2001 and 2015 appears to be driven by pockets of unvaccinated individuals, and not waning vaccine efficacy, according to an analysis by the Centers for Disease Control and Prevention (CDC).

Widespread measles vaccination allowed the United States to eliminate the endemic spread of measles in 2000, write Nakia Clemmons, MPH, from the CDC's Division of Viral Diseases, and colleagues in an article published in the October 3 issue of JAMA. However, small outbreaks sparked by travelers returning to the United States from countries where measles is endemic have continued to occur periodically.

Clemmons and colleagues analyzed confirmed US cases of measles between 2001 and 2015 to assess trends in US measles transmission. The analysis showed that the incidence of measles remains low at less than 1 case per 1 million Americans compared with the global incidence of 40 cases per million people.

During this period, however, the incidence in the United States nearly doubled, going from 0.28 per million in 2001 (95% confidence interval, 0.22-0.35) to 0.56 per million in 2015 (95% confidence interval, 0.48-0.65).

Moreover, the proportion of imported cases decreased from 46.6% in 2001 to 14.7% in 2015. At the same time, however, the number of cases occurring in vaccinated individuals declined from 29.6% to 20.2%. These concurrent trends suggest that increased numbers of unvaccinated individuals in the United States may be causing larger outbreaks after a smaller number of importations.

Rates of measles were highest among infants aged 6 to 11 months (5.44 per million) and toddlers aged 12 to 15 months (5.38 per million).

"The declining incidence with age, the high proportion of unvaccinated cases, and the decline in the proportion of vaccinated cases despite rate increases suggest that failure to vaccinate, rather than failure of vaccine performance, may be the main driver of measles transmission, emphasizing the importance of maintaining high vaccine coverage," the authors write.

The authors caution that difficulty verifying vaccination status for adults and possible changes in the way measles cases were reported during the period covered by the analysis may have affected the results.

Still, the trends are concerning in light of vaccine refusal in certain subpopulations in the United States and suggest that continued vigilance is needed to prevent endemic transmission from returning.

"Concerns about susceptibility pockets underscore the need for continued surveillance and rapid containment strategies," the authors conclude.

The authors have disclosed no relevant financial relationships.

JAMA. 2017;318:1279-1280. Abstract

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