Measles Elimination — Using Outbreaks to Identify and Close Immunity Gaps

Wednesday, 12th of October 2016

Measles Elimination — Using Outbreaks to Identify and Close Immunity Gaps

David N. Durrheim M.B. Ch.B. Dr.P.H.

N Engl J Med 2016; 375:1392-1393October 6 2016DOI: 10.1056/NEJMe1610620

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The Americas are on the eve of being formally verified as having eliminated endemic measles after the successful interruption of the transmission of measles in Brazil.1 This achievement provides empirical proof of the conclusion reached by an expert advisory committee convened by the World Health Organization (WHO) in 2010 that “global measles eradication is biologically technically and operationally feasible.”2 To capitalize on this opportunity for global good the Global Vaccine Action Plan enthusiastically targets the elimination of measles and rubella in at least five WHO regions by 2020.3 All six WHO regions have established goals to eliminate measles by 2020 or before.

Measles will not however go quietly. The elimination of measles the most infectious communicable disease of humans known (having a basic reproduction number — the average number of secondary cases generated by a primary case in a completely susceptible population — of 12 to 18) will require extraordinary immunization coverage to reduce the effective reproduction number below 1 which is necessary to achieve elimination.4 In almost every location in the world except for some small remote (island) communities 95% of each birth cohort will need to be immune to measles for a level of protection to be achieved that will preclude reestablishment of the transmission of endemic measles. The perpetually migratory nature of our global village will ensure that importations of measles test the integrity of community immunity in every country until measles is eradicated.

We need sensitive means for detecting any gaps in immunity; unfortunately records of routine immunization coverage are inadequate in many countries particularly historically. Serosurveys may assist in defining immunity profiles of a population but they are extremely resource intensive are logistically challenging to perform and difficult to interpret and rarely provide sufficient certainty and direction that is programmatically useful.

The article by Gastañaduy et al. in this issue of the Journal includes a detailed description of the measles outbreak in an Amish community in Ohio and provides compelling evidence of the value of measles outbreaks in identifying and characterizing immunity gaps that demand action.5Subpopulations or age cohorts that are vulnerable to measles because of inadequate immunity will be revealed by careful identification of the demographic characteristics of persons affected during a measles outbreak including their location age group and social cultural religious ethnic and other shared features. This descriptive demography and epidemiology may be sufficient in its own right to allow accurate targeting and engagement of vulnerable subpopulations as was the case with the Amish community. In situations in which the solution is less clear tools are available to assist in accurately defining the subpopulation at increased risk and elucidating strategies to most effectively increase immunization levels.6

The outbreak in the Amish community highlights the challenge posed by unimmunized or partially immunized young adult travelers who visit areas in which measles is still endemic and subsequently return home with the virus. Effective means for reaching this group who do not avail themselves of pretravel advice and immunization are proving to be elusive and more creative ways of linking immunization with visa issuance should be explored. This conundrum with respect to young adults is emerging in many countries in which the inadequate performance of historical immunization programs has resulted in a perverse legacy of unprotected older teenagers and young adults that provides a fertile opportunity for transmission of measles. A recent global meeting investigated the scale of the immunity gap among young adults and explored the array of approaches that countries were taking to counter this challenge and to close existing immunity gaps.7

As an aside it is inexcusable that relief workers are not appropriately immunized when the threat of vaccine-preventable infections is predictable in their destination country; in the case of the Amish community the workers traveled to the Philippines where measles remains endemic.

Every death from measles is a tragedy that should have been prevented.8 Effective tools to achieve the elimination of measles are already available and even more effective delivery methods such as the use of microarray patches are on the horizon. Every immunity gap should be filled. Because the measles virus has an uncanny ability to expose these immunity gaps we should capitalize on this phenomenon and allow a careful understanding of measles outbreaks to be our guide as we progress toward the ultimate goal of measles eradication.