Measles is in the news again. The epidemic that began at Disneyland in December has gotten a lot of media attention over the last several weeks. I've read a number of comments by different authors that refer to measles elimination from the U.S. in the past tense, as if the current outbreak means that measles is once again endemic in this country. It's not. Measles is still eliminated from the U.S. and the rest of the Americas.
The terms elimination and eradication are often used synonymously but, when speaking about disease control, they have different meanings. The classic definitions from the 1997 Dahlem Workshop are:
Elimination: Reduction to zero of the incidence of a specified disease in a defined geographical area as a result of deliberate efforts; continued intervention measures are required.
Eradication: Permanent reduction to zero of the worldwide incidence of infection caused by a specific agent as a result of deliberate efforts; intervention measures are no longer needed
- Dowdle, 1998
The risk of importation of a disease to a country or region from which it had been eliminated will be a risk until the disease is eradicated, which is why continued intervention measures are required. The last case of naturally-acquired smallpox occurred in Somalia in 1977. The World Health Assembly declared smallpox eradicated in 1980. Because there is no longer a risk of importation, smallpox vaccine is no longer used in the general population of the U.S. or any other country. In fact, routine smallpox immunization had not been recommended in the U.S. since 1971 (CDC, 2001), which explains why I have a smallpox vaccination scar and Holly does not.
Because there is a continuing risk of importation and less than 100% of the population will be protected against an eliminated disease, limited indigenous transmission can be expected. Elimination does not mean that the disease no longer occurs in a country or region that has been certified free of that disease. In 1983, Alan Hinman and colleagues wrote,
[Measles elimination] is stated in terms of indigenous measles since importations and measles will continue to occur. Success in achieving the goal will be demonstrated by the absence of any cases of measles in the United States that are not traceable within two generations of infection to a foreign source.
In that statement, generation refers to chains of transmission. A person infected from an imported source is a first generation transmission. Another person infected by that person is a second generation transmission.
In 2000, De Serres et al. wrote,
If elimination is perceived as the absence of secondary indigenous transmission, such outbreaks will erroneously lead to the conclusion that a vaccination program has not achieved this objective.
Measles elimination is therefore defined as the absence of endemic measles transmission.
Endemic measles transmission is the existence of any continuous indigenous chain of transmission of measles virus that persists for ≥1 year in any defined geographic area (e.g., the United States).
- Papania & Orenstein, 2004
Although there were 23 measles outbreaks in the U.S. last year, including a large outbreak among unvaccinated Amish, none of them resulted in sustained transmission for a year or longer.
Measles elimination in the United States was certified in 2000. Two years later, the World Health Organization Region of the Americas was certified free of endemic measles.
Over the last couple of weeks a number of people have made public statements that the Disneyland measles outbreak was due to importation of the virus by illegal immigrants from Mexico and/or other Latin American countries. The genotype of the measles virus in this outbreak is identical to the virus that caused a large measles epidemic in the Philippines. The B3 genotype is endemic in sub-Saharan Africa and has caused outbreaks in several other countries, including the United States (Rota, et al., 2011).
Visitors to Disneyland come from all over the world, including countries where measles has not been eliminated. Recent outbreaks of measles in the U.S. have been traced to unvaccinated U.S. citizens returning to this country from endemic countries and visitors who entered this country legally (CDC, 2013; 2014).
We don't know the source of this outbreak with absolute certainty, but it seems to me to be
absurd to suggest highly unlikely
that this outbreak is the result of an importation from a region of the world where
measles is eliminated and much more likely to have been brought to this country
by a tourist who entered this country legally.
To be continued.
Centers for Disease Control and Prevention. (2001). Vaccinia (smallpox) vaccine recommendation of the Advisory Committee on Immunization Practices (ACIP), 2001. Morbidity and Mortality Weekly Report, 50(10), 1-25. http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5010a1.htm.
Centers for Disease Control and Prevention. (September 13, 2013). Measles – United States, January 1 – August 2013. Morbidity and Mortality Weekly Report, 62(36),741-743. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6236a2.htm.
Centers for Disease Control and Prevention. (June 6, 2014). Measles – United States, January 1 – May 23, 2014. Morbidity and Mortality Weekly Report, 63(22), 496-499. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6322a4.htm.
De Serres, G., Gay, N. J,. & Farrington, C. P. (2000). Epidemiology of transmissible diseases after elimination. American Journal of Epidemiology, 151(11), 1039-1048.
Dowdle, W. R. (1998). The principles of disease elimination and eradication. Bulletin of the World Health Organization, 76(Supple. 2). 22-25.
Hinman, A. R. et al. (1983) Elimination of indigenous measles from the United States. Reviews of Infectious Diseases, 5(3), 538-545.
Papania, M. J. & Orenstein, W. A. (2004). Defining and assessing measles elimination goals. Journal of Infectious Diseases, 189(Supple. 1). doi:10.1086/381556.
Rota, P. A., Brown, K., Mankerta, A., Santibanez, S., Shulga, S., Muller, C. P., et al. (2011). Global distribution of measles genotypes and measles molecular epidemiology. Journal of Infectious Diseases, 204(Supple. 1). doi:10.1093/infdis/jir118.