Measles (also called rubeola) is a severe and highly contagious viral infection of the respiratory tract, although its most prominent symptom is a skin rash.
The measles virus spreads by direct contact with an infected person. Usually, the virus spreads via droplets of fluid from the person's respiratory tract. These droplets contain millions of virus particles that can infect another person, entering through the respiratory tract. Here, the virus incubates for one to two weeks before symptoms appear: fever, discomfort, sore throat, coughing, and finally a painful and itchy rash. After a few more weeks, the infection usually subsides. In a few cases, infection leads to pneumonia, brain damage, ear and sinus infections, convulsions, and sometimes death.
In developed countries, measles usually is not a fatal disease. In many developing countries, however, measles has a much higher mortality rate, accounting for 10 percent of all deaths in children under age 5.
People who have measles show a variety of symptoms, ranging from mild fever to severe skin rashes, to life-threatening seizures and infections. Doctors diagnose measles by the presence of Koplik's spots—tiny, white specks, surrounded by a red halo, that appear on the inside of the cheek, near the molars. Doctors can also use blood tests to check for antibodies against the measles virus.
Measles epidemics occur when the measles virus spreads rapidly through a susceptible population. Epidemics pose the greatest threat to unvaccinated people or people who have had only one dose of the vaccine and failed to develop antibodies against the virus.
Populations with high vaccination rates are less susceptible to epidemics. However, such populations can experience measles outbreaks in which three or more linked cases of the disease occur. Outbreaks are shorter in duration and more limited in transmission than epidemics.
The higher the percentage of unvaccinated people, the more susceptible a population is to an epidemic. The "epidemic threshold" is the point at which the percentage of unvaccinated people is high enough to risk an epidemic.
There are three kinds of immunity to measles: passive immunity, natural immunity, and immunity derived from vaccination. Infants born to mothers who have either had measles or been vaccinated are protected by maternal antibodies; that is, they have passive immunity. This protection lasts six months on average, and then the child becomes susceptible to measles. A person is naturally immune if he or she has had contact with the measles virus and has developed antibodies against it. People born before 1957 are considered naturally immune because of the high probability that they were exposed to the virus during childhood. People born after 1957 are considered immune if they have been fully vaccinated, have had a confirmed case of measles, or have had blood tests that confirm previous exposure to the virus. Full vaccination requires two doses of vaccine: one between the ages of 12 to 18 months, and the other between the ages of 4 to 6 years or 11 to 12 years. (The second dose helps catch the small number of people who do not become immunized by the first dose.)
During this century, there has been a dramatic decrease in measles epidemics. Prior to the development of the measles vaccine, 5.7 million people died each year from measles. (Some historians have suggested that measles might have contributed to the decline of the Roman Empire.)
In 1920, the United States had 469,924 measles cases and 7,575 deaths due to measles. From 1958 to 1962, the United States had an average of 503,282 cases and 432 deaths each year. (Measles reporting began in 1912; prior to this time, no statistics are available.) In large cities, epidemics often occurred every two to five years.
When the measles vaccine came on the market in 1963, measles began a steady decline worldwide. By 1995, measles deaths had fallen 95 percent worldwide and 99 percent in Latin America. In the United States, the incidence of measles hit an all-time low in 1998, with 89 cases and no deaths reported.
There have been several epidemics in the United States since 1963: from 1970 to 1972, 1976 to 1978, and 1989 to 1991. The epidemic of 1989-1991 claimed 120 deaths out of a total of 55,000 cases reported. Over half of the deaths occurred in young children.
In 1997, the Centers for Disease Control and Prevention (CDC) reported a total of 138 cases of measles in the United States. Most of these outbreaks probably began when an infected person from another country (specifically Germany, Italy, Switzerland, Brazil, and Japan) entered the United States. The virus subsequently spread through the population, with the longest chain of transmission lasting five weeks. Children were most affected by these outbreaks: 29 percent of cases were children 1-4 years old; 28 percent were children 5-19; 26 percent were adults 20-39. In addition, unvaccinated people accounted for 77 percent of cases; people who received only one dose of vaccine accounted for 18 percent of cases; and people who received the full two doses of vaccine accounted for 5 percent of cases. (These statistics demonstrate that a small percentage of people fail to develop immunity after one or even two doses of vaccine.)
In 1998, the United States had only 89 cases and no deaths due to measles. Measles cases clustered in a few states. Arizona, California, Florida, Massachusetts, Minnesota, New York, Pennsylvania, South Dakota, and Texas reported 64 percent of measles cases in 1997. Most of these cases were from foreign visitors who brought the virus with them or from U.S. citizens who contracted the virus while traveling abroad. These patterns suggest that there is no established measles virus circulating in the United States.
The Western Hemisphere (countries in the Americas and the Caribbean) has the lowest incidence of measles worldwide, with only 2,109 cases reported in 1996. However, low rates of vaccination among some populations resulted in several outbreaks: in 1997 in Brazil (51,000 cases); in 1998 in Argentina (3,000 cases and 11 deaths); and in 1998 in Bolivia (111 cases). Children under 4 were most commonly affected by these outbreaks. An outbreak at a Canadian university also suggested that low immunization rates among students had left an opening for the measles virus. In all cases, gene sequencing indicated that the virus had come from a foreign source.
According to the World Health Organization (WHO), there were 31 million cases of measles in 1997, resulting in almost 1 million deaths. (These figures are estimates because only a fraction of measles cases worldwide are actually reported.) The majority of these cases occurred in Africa, followed by Asia, India, and the Middle East. In fact, in 1997 roughly 99 percent of all measles deaths occurred in developing countries.
In 1990, measles was the eighth leading cause of death. In 1997, it was the sixth leading cause. According to some analyses, this represents a greater loss of life than that caused by AIDS and almost as great a loss as that caused by malaria. The majority of deaths occur among young children. In developing countries, measles accounts for 10 percent of all deaths in children under age 5.
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