Measles in the United States
1- The control and virtual elimination of measles in the United States is a public health success that has provided a model for immunization programs in other parts of the developed world. Before measles vaccination was introduced in the United States in the mid-1960s, more than half a million cases of measles were reported each year. Once a vaccine was developed,public health officials set out to use it to control the disease, envisioning eventual global eradication. By the mid-1970s, fewer than 50,000 cases were being reported annually in the United States, but a severe outbreak in Los Angeles in 1977 reminded authorities how tenuous the control of measles was. Compulsory immunization of school children followed — the first in a series of steps that ledto the interruption of the transmission of measles in the United States. The most effective of these steps was probably the introduction of a second dose of measles vaccine in 1989, again in response to a large outbreak of disease. At a meeting in March 2000, leaders in the field concluded that "measles is no longer endemic in the U.S.
2- Measles is an acute infectious disease that can affectpersons of all ages but is most severe in young infants and adults. Person-to-person transmission occurs through direct contact or droplet spread, and there is some evidence that severity is related to infective dose, so that those who acquire the disease from close household contact have the most severe disease. Infected persons can infect others even before the characteristic rash appears, creatingample opportunity for people who are incubating the disease to travel from a country where the virus is circulating, such as Romania or India (the latter being the probable origin of an outbreak in Boston in the spring and early summer of 2006), and bring the virus with them.
3- At the time of the 2005 outbreak, the level of vaccination coverage in Indiana was 98 percent for the second dosedelivered to school-age children. In such a setting, it can be expected that the number of vaccinated persons who do not acquire protective immunity will be small, so the primary group of persons who are susceptible to the disease will consist of those who are unvaccinated. An important subgroup will be children who are too young to be vaccinated. These infants become susceptible when maternalimmunity wears off, and they remain susceptible until they are immunized.
4- The Indiana outbreak was striking for a number of reasons. All but two of the cases occurred in unvaccinated hosts, indicating that it was the failure to vaccinate rather than vaccine failure that caused the problem. Five hundred people were reportedly present at the church gathering where the initial U.S. transmissionoccurred. Church officials estimated that 10 percent of them were unvaccinated, but it is unclear how they knew this or how accurate their estimate was. It seems unlikely that the person with the index case of measles could have directly infected 32 percent of all susceptible persons at the meeting, and It would suggest that the number of unvaccinated persons there may have been higher. But even 10percent is a high proportion in a state with 98 percent coverage for the second dose of measles vaccine, despite the fluctuations in coverage that may have occurred during the lifetimes of those who became infected, most of whom were younger than 20 years old. Of the two vaccinated persons who acquired measles in Indiana, one was 34 years old and had been vaccinated only once in infancy. We cannottell whether this case represents a primary vaccine failure (a failure of the vaccine to induce immunity) or a secondary vaccine failure (the result of waning immunity). The severity of the case suggests the former, since secondary vaccine failures tend to produce mild disease.
5- More than 50,000 cases of mumps have been identified in the United Kingdom during the past few years, and more...
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