The first outbreak of polio in the United States in 26 years occurred earlier this fall in an Amish community in central Minnesota, state and federal health officials reported yesterday.
Four children have been infected with the virus, although none has become paralyzed. The Amish typically decline to vaccinate their children. The last large outbreak of polio occurred in numerous Amish communities in several states in 1979.
The outbreak poses little threat to children outside the Amish community. About 98 percent of Minnesota's children are vaccinated against polio, said Harry Hull, the state epidemiologist.
The outbreak was discovered by chance on Sept. 29 after the first child -- a 7-month-old infant with a severe immune deficiency disease -- was tested for another problem in August. Yesterday's announcement reveals the microbe is circulating among healthy children in the isolated community, which has about 200 people in 24 families.
Polio causes paralysis in about one in every 200 infections.
The virus that all four children are carrying is derived from the oral polio vaccine. That vaccine has not been used in the United States since 2000, in part because it causes paralysis in about one of every 13 million doses administered. American children now get an injected vaccine, which also prevents infection.
The oral vaccine, which is still used in most places in the world, is made of a live but severely weakened strain of polio virus. The vaccine virus can be passed person to person, although it rarely becomes part of a prolonged "chain of transmission" because most people in a population are vaccinated and cannot be infected.
Occasionally, however, a vaccine strain circulates for years, passed from one unvaccinated child to another. When that happens, it undergoes genetic mutation that can restore the dangerousness of the "wild" virus.
Jane Seward, a vaccine expert at the federal Centers for Disease Control and Prevention, said genetic fingerprinting of the Minnesota strain shows it is about 2.3 percent different from the vaccine strain. This suggests it has been circulating for a little more than two years.
Where it was circulating, however, is a mystery. Hull said it is likely the virus was imported from a country where the oral vaccine is still in use, but the Amish have little contact with people outside their community. The first infected child had no known exposure to foreigners.
Public health officers are going door to door offering polio vaccine and requesting stool samples of all children. About 35 samples have been collected and 32 tested. Fewer than 20 children have been vaccinated, Hull said.