In honor of National Infant Immunization Month, we’re highlighting diseases this week that are preventable by vaccination of infants. Polio is one of these diseases, and it has a remarkable background: it was scarcely visible through much of human history, was epidemic from the early- to mid-20th century, and is nearly eradicated today. The development of the polio vaccine was an important U.S. cultural phenomenon, involving the monetary contributions of millions of citizens, scientific breakthroughs by medical researchers, and the largest clinical trial ever conducted.
Few diseases frightened parents more in the early part of the 20th century than polio did. Polio struck in the warm summer months, sweeping through towns in epidemics every few years. Though most people recovered quickly from polio, some suffered temporary or permanent paralysis and even death. Many polio survivors were disabled for life. They were a visible, painful reminder to society of the enormous toll this disease took on young lives.
It is likely that polio plagued humans for thousands of years before it emerged in epidemic form. An Egyptian carving from around 1400 BCE depicts a young man with a leg deformity similar to one caused by polio. Polio probably circulated in human populations at low levels and was a relatively uncommon disease for most of the 1800s.
Polio reached epidemic proportions in the early 1900s in countries with relatively high standards of living, at a time when other diseases such as diphtheria, typhoid, and tuberculosis were declining. Indeed, many scientists think that advances in hygiene paradoxically led to an increased incidence of polio. The theory is that in the past, infants were exposed to polio, mainly through contaminated water supplies, at a very young age. Infants’ immune systems, aided by maternal antibodies still circulating in their blood, could quickly defeat poliovirus and then develop lasting immunity to it. However, better sanitary conditions meant that exposure to polio was delayed until later in life, on average, when a child had lost maternal protection and was also more vulnerable to the most severe form of the disease.
The Race to the Polio Vaccines
The National Foundation for Infant Paralysis, also known as the March of Dimes, funded a great deal of research that led to the polio vaccines. They raised money through national public awareness campaigns and involved countless parents, children, and other community members in their efforts.
One important step in the race to the vaccines was David Bodian and Isabel Morgan’s 1949 discovery of three immunologically distinct types of poliovirus. A vaccine would need to provide protection against all types of polio. In Boston that same year, the team of John Enders, Thomas Weller, and Frederick Robbins showed that they could grow polioviruses in non-nervous tissue—namely human embryonic skin and muscle tissue. This landmark finding would reduce reliance on using live monkeys for growing and testing virus. No longer would polio research be restricted to facilities that could house large numbers of experimental animals. Enders’s findings would lead the way to simpler, less expensive methods of producing large quantities of virus for study and eventually vaccine production.
Jonas Salk’s inactivated polio vaccine was successfully tested in an enormous field trial in the spring and summer of 1954. After widespread polio vaccination began in 1955, and the replacement of the inactivated Salk-type vaccine with the live Sabin-type vaccine, polio incidence declined dramatically in the United States. From a high of about 57,000 polio cases reported in 1952, the 1970s saw only 17 cases of polio imported from other countries, and 30 cases of paralytic polio from circulating (endemic) wild-type poliovirus. The last outbreak of circulating poliovirus in the United States occurred among Amish communities in Pennsylvania and the Midwest. During the 1980s, a few cases of polio were imported into the United States, probably from Mexico. Occasional cases of polio resulting from use of the oral (live attenuated) vaccine occurred throughout these years, until the United States returned to using the inactivated polio vaccine in the late 1990s.
On August 20, 1994, the Pan American Health Organization reported that three years had passed since the last case of wild polio in the Americas. A three-year-old Peruvian boy, Luis Fermín, had the last recorded case there. Polio was declared eliminated from the Western Hemisphere.
Today, polio continues to circulate in a handful of countries, with occasional spread to neighboring countries. (Endemic countries are Afghanistan, India, Nigeria, and Pakistan.) Vigorous vaccination programs are being conducted to eliminate these last pockets. Polio vaccination is still recommended worldwide because of the risk of imported cases.
In the United States, children are recommended to receive the inactivated polio vaccine at 2 months and 4 months of age, and then twice more before entering elementary school.
For more information on the history of polio and the polio vaccines, see our polio timeline.
The Centers for Disease Control and Prevention. Epidemiological notes and reports follow-up on poliomyelitis. December 19, 1997 / 46(50);1195-1199. http://www.cdc.gov/mmwr/preview/mmwrhtml/00050435.htm
The Centers for Disease Control and Prevention. Summary of Notifiable Diseases, United States, 1993.
Webber R. Communicable disease epidemiology and control: a global perspective. CABI, 2009.