Why polio was so feared
Polio, or poliomyelitis, is an infection caused by a virus that spreads mainly through contaminated water, food and hands. Most infections cause mild illness or none at all, but in a small share of cases the virus attacks nerve cells in the spinal cord and brainstem, causing paralysis. Some patients could not breathe on their own and were treated in large mechanical “iron lung” respirators.
For much of history polio was a sporadic, mostly unnoticed disease. In the late nineteenth and early twentieth centuries, as sanitation improved, babies were exposed to the virus less often, and a growing number of people met it for the first time as older children or adults. Large epidemics followed, including a major outbreak in New York City in 1916. Summer became the dreaded season, and parents kept children away from pools and crowds. In the United States, reported cases peaked in the early 1950s, with more than 50,000 cases in 1952.
Early science
In 1908 Karl Landsteiner and Erwin Popper showed that the disease could be passed to monkeys by material that passed through filters fine enough to hold back bacteria, which pointed to a virus. Ideas from germ theory had already established that specific microbes cause specific diseases, but viruses were still poorly understood, and they could not be seen with the microscopes of the day.
A practical problem held research back for decades: scientists could only grow poliovirus in nerve tissue, which made large-scale work difficult and costly, and which carried safety concerns for any vaccine made that way. A few early vaccine attempts in the 1930s were unsuccessful and are remembered mainly as cautionary examples.
Money and a public campaign
One reason the research moved quickly was money. Franklin D. Roosevelt, who had been paralyzed by an illness in 1921 that was long diagnosed as polio, helped lead the creation of the National Foundation for Infantile Paralysis in 1938. Its fundraising campaign, later known as the March of Dimes, collected small donations from millions of Americans. That money paid for patient care and for laboratory research, including the work that led to the first vaccine.
The tissue culture breakthrough
In 1949 John Enders, Thomas Weller and Frederick Robbins reported that poliovirus could be grown in cultures of non-nerve human tissue. That made it possible to produce virus in quantity, to study its types, and to test vaccine candidates more safely. Their work earned a Nobel Prize in 1954. Researchers also established that there were three distinct types of poliovirus, so a useful vaccine would need to protect against all three.
The Salk vaccine
Jonas Salk, working at the University of Pittsburgh with support from the National Foundation, developed an inactivated vaccine. The virus was grown in cell cultures and then treated with formaldehyde so that it could no longer cause disease while still prompting the immune system to respond. It was given by injection using a hypodermic syringe, in a series of doses.
A very large field trial in 1954 tested it on children, and on April 12, 1955, the results were announced and the vaccine was licensed in the United States. The news spread by television broadcasting and newspapers, and many people treated the day as a celebration. The trial itself, and how safety and effectiveness were judged, has its own history, and the results were generally reported as showing that the vaccine protected against paralytic polio, with protection varying by poliovirus type.
The Cutter incident
Only days after licensing, cases of polio were linked to vaccine from one manufacturer, Cutter Laboratories, in which some virus had not been fully inactivated. Hundreds of children became ill, some were paralyzed, and several died, according to most accounts. Vaccination was briefly paused, and the federal government tightened manufacturing and testing rules. The episode is often cited in the history of vaccine regulation as a reminder that production quality matters as much as the underlying science.
The Sabin vaccine
Albert Sabin took a different route. His oral vaccine used live viruses weakened, or attenuated, by growing them repeatedly in cultured cells so that they replicated in the gut without usually causing paralysis. The advantages were practical: it could be given by mouth, often on a sugar cube or as drops, did not need needles or trained injectors, and it spread immunity in the gut, which could also protect people around those vaccinated.
Because the United States had already adopted Salk’s vaccine, Sabin’s candidates were tested widely overseas, including in the Soviet Union, where researchers such as Mikhail Chumakov and Anatoly Smorodintsev worked with them in large campaigns involving millions of people. Oral vaccines were licensed in the United States in the early 1960s, first for single poliovirus types and then as a combined vaccine covering all three.
Two vaccines, different trade-offs
The two vaccines are often compared:
| Salk (inactivated) | Sabin (oral, live attenuated) | |
|---|---|---|
| Given by | Injection | Mouth |
| Virus | Killed | Weakened but live |
| Main strength | Cannot cause vaccine-related polio | Cheap, easy, helps stop spread |
| Main drawback | Needs injection and trained staff | Very rarely, the weakened virus can revert and cause paralysis |
Many countries used oral vaccine for decades because of cost and ease, and then moved back toward injected, inactivated vaccine as wild polio disappeared and the rare vaccine-related cases became a larger share of the remaining risk. Choices differ by country and over time.
Bringing polio under control
In wealthy countries, the number of cases fell sharply within a few years of widespread vaccination. The United States, for example, reported far fewer cases by the early 1960s than at the 1952 peak. In 1988 the World Health Assembly set a goal of global eradication, and the effort that followed combined routine immunization, mass campaigns and disease surveillance. The Americas were certified free of wild polio in 1994. Two of the three wild poliovirus types have since been declared eradicated.
Eradication has proved harder than hoped. Wild poliovirus type 1 has remained in a small number of places, and in under-vaccinated communities weakened vaccine virus can occasionally circulate and regain the ability to cause paralysis. Progress has also been affected by conflict and by difficulty reaching some populations. The numbers change from year to year, so current status should be checked with health authorities.
Human cost and legacy
Polio survivors, some of whom live with lasting weakness, used braces, crutches and powered wheelchairs, and many later developed new fatigue and weakness decades afterward. The campaign also changed how medical research is funded and tested: public donations, large controlled trials and government oversight of manufacturing all became more established. This article is a history only, and it is not medical advice; questions about vaccination belong with a doctor or public health agency.