Measles Elimination in the Americas
The World Health Organization Region of the Americas has achieved a milestone in disease elimination – the Pan American Health Organization (PAHO) on September 27, 2016, declared the Americas to be free of endemic measles. In the pre-vaccine era, the WHO estimates that measles killed 2.6 million children per year globally, and so measles elimination has done a great deal to combat a major threat to child health.
The Region of the Americas is the first of the six WHO regions to eliminate transmission of measles. The United States was certified measles-free in 2000, and the last cases of endemic measles were reported in other countries of the region in 2002. The International Expert Committee for Documenting and Verifying Measles, Rubella, and Congenital Rubella Syndrome Elimination in the Americas was responsible for collecting reports from region countries to certify that measles has in fact been eliminated. Polio (certification in 1994), rubella (2015) and smallpox (1971) have been eliminated from the region as well, and, of course, smallpox has been eradicated globally (certification in 1980).
The elimination certification means that populations in the Americas have achieved herd immunity via high measles vaccination rates, preventing the ongoing transmission of measles in the region. Measles outbreaks may still occur, but if current practices continue, measles infections will result only in transmission from imported measles cases entering the Americas from other WHO regions. Of course, as the 2015 measles outbreaks stemming from exposures at Disneyland in Southern California demonstrate, imported measles cases can achieve spread through undervaccinated populations even when overall vaccination rates are high. In the Disneyland example, cases were limited to under 200 and the spread of measles was contained.
According to the Global Measles and Rubella Strategic Plan 2012-2020, WHO aims to achieve measles elimination and rubella elimination or control in at least five of its six regions by 2020. Already, though, targets set in this plan have fallen short. Measles elimination in the Western Pacific Region was targeted for 2012 and for 2015 for the Eastern Mediterranean and European regions, but none of these regions has met its goal. (The Southeast Asia region has not adopted a measles elimination goal, and the African region is targeted to eliminate measles by 2020.)
I spoke with Stanley A. Plotkin, MD, developer of the rubella component of the measles, mumps, rubella vaccine about prospects for the future. He noted that elimination of measles from the Americas is a signal accomplishment, considering the high infectiousness of the measles virus. Plotkin predicted “It will be more difficult to eliminate measles from other parts of the world because of the poor uptake of vaccine in many European countries and the difficulties with reaching all children in Africa and Asia.” Indeed, estimated global two-dose measles containing vaccine coverage (MCV2) for children was only 56% worldwide in 2014, and about 20% of the world’s countries had not yet added a second dose of measles vaccine to their childhood immunization program. In 2013, MCV2 coverage in the different WHO regions was estimated as follows:
- 7% in the Africa region
- 53% in the South-East Asia region
- 65% in the Eastern Mediterranean region
- 82% in the European region
- 92% in the Western Pacific region.
Interestingly, MCV2 coverage in the Americas was estimated at only 46%, and yet the region was able to achieve elimination before the others. This may be due to consistently high regional MCV1 coverage, with region-wide rates of greater than 90% since 1998. This high MCV1 coverage was achieved via many years of programmatic catch-up, follow-up, and speed up immunization campaigns, as described in Castillo-Solorzano et al. (2015)
These immunization activities resulted in a precipitous drop in measles incidence–from 320 cases per million population in the years 1980-86, to 170/million in 1987-1994, to 13/million in 1995-2002, to .2/million in 2003-2009 (remember that incidence figures in the elimination era include non-endemic transmission from imported cases). In comparison, in the period 1 August 2015 -- 31 July 2016, Romania reported 17 cases/million, Italy reported 10/2 cases/million, and Ireland 8.9 cases/million. And in 2012, incidence was as high as 964.7/million in Zambia.
Clearly, much work remains to improve measles vaccination coverage in order to achieve the goal of global measles eradication. The Measles & Rubella Initiative is an excellent source of information about collaborative global efforts to end measles.