According to Centers for Disease Control (CDC), the U.S. will likely see more measles cases this year than in any year of the past decade.

At the present pace, about 270 cases will be reported in 2011. Typically, the country sees only about 50 cases a year. Even so, the number of cases in the U.S. remains low compared to most other countries.

The U.S. rate is low, thanks to vaccinations. Two doses of measles-mumps-rubella (MMR) vaccine are routinely recommended for all children. The first dose is given around a child’s first birthday and the second at least a month later but usually around age four. After the recommended two doses, the chance of getting measles (or mumps or rubella) drops below one percent. The vaccinations are believed to last a lifetime.

This year, Europe has been hit hard by measles. More than 6,500 cases have been reported in 33 nations. The situation in France has been particularly bad. According to international health officials, the problem is linked to the failure to vaccinate all children.

Measles is highly contagious. The virus spreads easily through the air. After an infected person leaves a closed room, contagious droplets can linger for up to two hours. According to Dr. Greg Wallace, who leads the MMR team at the CDC, “Measles is really the most contagious of the vaccine-preventable diseases. It has a knack for finding those who have not been vaccinated.” Up to 90 percent of unvaccinated people who are exposed will get the disease.

Measles’ most common symptoms are fever, runny nose, cough, eye inflammation and a rash all over the body. Victims are contagious from four days before until four days after the rash appears.

Almost all the cases in the current U.S. breakout have resulted from exposures that initiated overseas. In Florida, five cases were linked to an international helicopter show held in Orlando, and three cases in Gainesville were traced to a traveler who had recently been in India. Utah reported nine cases linked to someone who had been in Poland. Twenty-one cases in Minnesota apparently were the result of a local child who developed symptoms after returning from a trip to Kenya.

All these outbreaks were limited by the “herd immunity” in the U.S. If 95 percent of the population is vaccinated (herd immunity), measles cannot find enough susceptible victims to keep spreading, and it dies out on its own. Because it is so contagious, without herd immunity, measles would spread broadly throughout the population.

Other countries lack herd immunity. In developing countries vaccination rates are low mostly due to inadequate resources and health care systems that do not reach every family. In Europe, the rate is low partly in reaction to now-debunked speculation that MMR vaccinations cause autism. The speculation arose from a 1998 article in The Lancet, a respected British medical journal. Later, it was discovered that the article’s author, a physician, manipulated evidence and violated ethical standards. The article was retracted, but the scare developed an internet life of its own. In Britain, after it gained momentum, the MMR immunization rate dropped to 83 percent in 2007, and the country endured almost 1,000 measles cases. In the U.S., the misinformation had less impact, despite health officials’ fears that it might. The CDC, the American Academy of Family Physicians, the Institute of Medicine and the U.S. Academy of Sciences all thoroughly reviewed the evidence and found no connection whatsoever between immunization and autism.

In addition to MMR, a variety of other vaccinations are recommended for children and adults, and many are required by state authorities and school districts before children can enter school. At greatest risk are preschool children whose parents have not made sure that they are properly immunized.

CDC-recommended vaccine schedules are organized for birth to six years (Spanish) and children and teens(Spanish). Schedules for overseas travel are also available.

[Steve Clark]