Salk Polio Vaccine (1955): The Field Trial That Beat Polio

The Salk polio vaccine was the first effective vaccine against poliomyelitis, a viral disease that paralysed or killed tens of thousands of people each year in the mid-20th century. Developed by Jonas Salk at the University of Pittsburgh, it was tested in 1954 on about 1.8 million children in the largest medical trial ever held at that time, and declared safe and effective on 12 April 1955. Within a few years polio cases in the United States had fallen dramatically, beginning a global campaign to eradicate the disease.

Background: the fear of polio

Polio is caused by a virus that spreads mainly through contaminated food, water and hands. Most infections cause no symptoms or a mild illness, but in a small fraction of cases, roughly one in 200, the virus attacks the nerve cells of the spinal cord and causes paralysis. When the muscles used for breathing were affected, patients could survive only inside an “iron lung”, a large machine that pumped air in and out of their chests.

Paradoxically, polio epidemics grew worse as sanitation improved in the late 19th and 20th centuries. Babies who once met the virus early, while still protected by their mothers’ antibodies, now met it later in childhood. Summer outbreaks closed swimming pools and cinemas, and parents lived in fear. The worst American year was 1952, with about 58,000 reported cases, more than 3,000 deaths and over 21,000 people left with some paralysis.

Polio’s most famous victim was Franklin D. Roosevelt, paralysed in 1921, who later became US president. In 1938 he founded the National Foundation for Infantile Paralysis, led by his former law partner Basil O’Connor. Its fundraising campaign, the March of Dimes, collected small donations from millions of Americans to pay for patient care and research.

The science behind the vaccine

Two discoveries made a vaccine possible. In the late 1940s a large typing programme funded by the National Foundation, in which Salk took part, confirmed that there are three distinct types of poliovirus, so an effective vaccine needed to protect against all three. In 1949 John Enders, Thomas Weller and Frederick Robbins in Boston showed that poliovirus could be grown in cultures of non-nervous tissue. This allowed the virus to be produced in large quantities, and the three won the 1954 Nobel Prize in Physiology or Medicine.

Jonas Salk, born in New York City in 1914, had worked on influenza vaccines with Thomas Francis Jr. at the University of Michigan. At Pittsburgh from 1947, he chose a “killed-virus” approach. He grew the three types of poliovirus in cultures of monkey kidney cells and then inactivated them with formaldehyde, so that they could no longer cause infection but still stimulated the body to make protective antibodies.

Many virologists, notably Albert Sabin, believed that only a live but weakened virus could give lasting immunity, and they criticised Salk’s approach. In 1952 Salk tested his vaccine on children at two institutions near Pittsburgh, the D. T. Watson Home for Crippled Children and the Polk State School. In 1953 he vaccinated himself, his wife and their three sons. The results showed that the vaccine raised antibody levels without causing illness.

The 1954 field trial

The National Foundation decided to test the vaccine on a vast scale. To ensure that the results would be trusted, it asked Thomas Francis Jr. to run the evaluation independently from a centre at the University of Michigan. Francis insisted on a rigorous design in which, in many areas, children would be randomly given either the vaccine or a placebo, with neither the families nor the doctors knowing which, a method now called a double-blind trial.

The trial began on 26 April 1954, when six-year-old Randall Kerr received an injection at Franklin Sherman Elementary School in McLean, Virginia. Over the following months about 1.8 million children, most of them in the first three grades of school, took part in 44 US states, as well as in parts of Canada and Finland. They became known as the “Polio Pioneers”.

In the placebo-controlled areas, roughly 650,000 children received injections of vaccine or placebo. In other areas, second-graders received the vaccine while first- and third-graders were observed as controls. The operation relied on about 20,000 doctors and public health officers, some 64,000 school staff and around 220,000 volunteers. Francis’s team then spent almost a year tracking every case of polio among the participants.

The announcement of 12 April 1955

Francis presented his report on 12 April 1955, the tenth anniversary of Roosevelt’s death, at Rackham Hall at the University of Michigan in Ann Arbor. Hundreds of reporters attended, and the event was broadcast to thousands of doctors in cinemas across the country. The vaccine was found to be about 80–90 per cent effective against paralytic polio, and safe.

Church bells rang, and newspapers carried the news in their largest headlines. That afternoon the US government licensed the vaccine, and Salk became a national hero. In a television interview with the broadcaster Edward R. Murrow, asked who owned the patent, Salk replied that there was none and asked, “Could you patent the sun?”

The Cutter incident

The celebration was soon shaken. Within days of licensing, children vaccinated with batches made by Cutter Laboratories of Berkeley, California, began developing polio. The company’s inactivation process had failed to kill all the virus in some lots. About 120,000 doses of faulty vaccine were given.

The consequences were serious. Around 40,000 children developed mild, short-lived polio, about 200 were left with some degree of paralysis and 10 died. The virus also spread to family members and contacts. The national programme was briefly halted in May 1955 while all manufacturers’ procedures were reviewed.

The Cutter incident led to tighter federal oversight, including a strengthened Division of Biologics Standards, and to improved safety testing. It also established that manufacturers could be held liable for harm caused by vaccines, a principle that shaped the vaccine industry for decades.

Consequences: polio in retreat

Despite the setback, vaccination resumed and spread quickly. In the United States the number of polio cases fell from about 29,000 in 1955 to roughly 5,500 in 1957 and fewer than 1,000 by 1962. Many European countries began their own programmes from 1955 and 1956, and Denmark’s state serum institute produced the vaccine for Scandinavia.

Albert Sabin’s oral vaccine, made from live weakened virus, was tested on millions of people in the Soviet Union in 1959 with the help of the virologist Mikhail Chumakov, and was licensed in the United States in 1961–1962. Given as drops or on a sugar cube, it was cheap, easy to administer and gave strong immunity in the gut, so it largely replaced Salk’s vaccine in many countries. Because the live vaccine very rarely causes paralysis itself, the United States switched back to inactivated vaccine in 2000, as have many other countries.

In 1988 the World Health Assembly launched the Global Polio Eradication Initiative, following the success of the campaign against smallpox. Cases of wild polio have since fallen by more than 99 per cent. Wild poliovirus types 2 and 3 have been declared eradicated, and wild type 1 remains endemic only in Pakistan and Afghanistan.

Myths and facts

• Myth: Salk’s vaccine was given on a sugar cube. The sugar cube belongs to Sabin’s oral vaccine. Salk’s vaccine was always given by injection, usually in a series of doses.
• Myth: Salk alone gave away a fortune by refusing a patent. Salk did not seek a patent, and his famous remark reflected genuine conviction. But lawyers for the National Foundation had already concluded that the vaccine, built on earlier published methods, would probably not have qualified for one.
• Myth: The 1955 vaccine ended polio. It brought cases down sharply in wealthy countries, but global eradication has required decades of work, mostly with oral vaccine, and is still not complete.

Key people

• Jonas Salk (1914–1995): developed the inactivated vaccine and later founded the Salk Institute for Biological Studies in La Jolla, California.
• Thomas Francis Jr. (1900–1969): epidemiologist who designed and evaluated the 1954 field trial.
• Basil O’Connor (1892–1972): president of the National Foundation, who funded and drove the vaccine effort.
• Albert Sabin (1906–1993): developer of the rival oral vaccine.
• John Enders, Thomas Weller and Frederick Robbins: scientists whose method of growing poliovirus made vaccine production possible.

Timeline

• 1938, 3 January: Roosevelt founds the National Foundation for Infantile Paralysis.
• 1949: Enders, Weller and Robbins grow poliovirus in tissue culture.
• 1952: The worst US polio epidemic; Salk begins testing his vaccine near Pittsburgh.
• 1954, 26 April: The national field trial begins in McLean, Virginia.
• 1955, 12 April: Thomas Francis Jr. announces the vaccine safe and effective; it is licensed the same day.
• 1955, late April: Cases linked to Cutter Laboratories vaccine are reported.
• 1961–1962: Sabin’s oral vaccine is licensed in the United States.
• 1988: The Global Polio Eradication Initiative is launched.

Why it matters

Salk’s vaccine ended the summer terror of polio in much of the world and showed how science, public fundraising and a carefully designed trial could defeat a disease. Its field trial set new standards for testing medicines, the Cutter incident shaped modern vaccine regulation, and the campaign it began has brought polio to the brink of eradication, building on a tradition that runs back to Edward Jenner’s smallpox vaccine and forward to the eradication of smallpox.