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.
Previous posts:
References:
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.
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