Jonas Salk changed the world when he developed a polio vaccine in 1955. Cases of polio dropped from 21,000 in 1952 to fewer than 100 a decade later. Since 1979, nobody in the United States has been infected with the disease. The exception to this success story is polio contracted from the vaccine; there have been about nine cases per year of this type of polio. There are two types of polio vaccine, a live and an inactivated virus, and, though it was more likely to give a person polio, most people got the live virus from 1963 to 2000. In 2000, the government changed its policy, saying that the inactivated virus should be used. As part of a polio surveillance report done every decade, researchers from the Centers for Disease Control and Prevention looked at the impact of that switch on cases of vaccine-associated polio.

What the researchers wanted to know: Did the switch from the live polio vaccine to one that used an inactivated virus reduce vaccine-associated polio in the United States?

What they did: The researchers used data collected by the Centers for Disease Control and Prevention from local health clinics about cases of polio in the United States between 1990 and 2003. The researchers then correlated those numbers with data from the National Immunization Survey from 1995 to 2002. That survey, done randomly by telephone, includes questions about children's immunizations.

What they found: Fifty-nine cases of vaccine-associated polio were confirmed in the United States between 1990 and 2003, the last occurring in 1999. Some children had gotten both the inactive and the live virus, but no one who had the inactive vaccine shot before the live vaccine came down with polio. Since the inactive vaccine shot became the norm in 2000, no cases of vaccine-associated polio have been reported in the United States.

What it means to you: The inactive polio vaccine has lowered the risk of contracting polio in the United States, either from the natural virus or from the shot, to nearly zero. Polio has not been wiped out all over the world, however—Nigeria reported 760 cases in 2004. The World Health Organization currently has a program that is trying to eradicate polio worldwide and has achieved that goal in all but six countries—Nigeria, India, Pakistan, Niger, Afghanistan, and Egypt.

Caveats: For 1993 to 1996, the researchers did not know how many children received a polio vaccination. They assumed that every child born in the United States got a shot, which is clearly a stretch. However, available data from other sources suggest that more than 98 percent of children did receive at least part of their polio vaccination, so the researchers' data are probably not too far off.

Find out more: Polio has been eradicated in the United States but can still be a risk for people traveling to certain parts of the world. Find out more about that risk from the Centers for Disease Control and Prevention.

Read the article: Alexander, L.N. et al. "Vaccine Policy Changes and Epidemiology of Poliomyelitis in the United States." Journal of the American Medical Association. Oct. 13, 2004, Vol. 292, No. 14. pp. 1696–1701.

Abstract online: http://jama.ama-assn.org