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Two polio vaccines are used throughout the world to combat polio. The first was invented by Jonas Salk and first tested in 1952 and announced to the world by Salk on April 12, 1955. It consists of an injected dose of killed polio virus. Thereafter, Albert Sabin produced an oral polio vaccine using live but weakened (attenuated) virus. Human trials of Sabin's vaccine began in 1957 and it was licensed in 1962.

When the live-virus Sabin vaccine was developed, it spread in popularity for several reasons. First, it can 'infect' other, non-vaccinated individuals with whom the vaccinated person has close contact and confer some immunity to them. Second, because the oral vaccine acts in the gut, it confers immunity there and reduces the spread of the wild virus. The injected vaccine, acting through the bloodstream, immunizes the individual but does not reduce his ability to spread the wild virus. Third, the live-virus vaccine is cheaper than the killed-virus vaccine. Finally, the oral vaccine is easier to administer than the injected vaccine, so patients are more likely to complete the vaccination series and attain full immunity. Though Salk's vaccine had reduced the incidence of polio to a tiny fraction of what it was in the early 1950s, Sabin's vaccine was considered superior for these reasons and became the standard treatment. The killed-virus vaccine immunized people against the effects of the virus, but the virus could still spread from person to person. It was the live-virus vaccine that enabled the complete eradication of the wild polio virus in the United States.

The major disadvantage of the live-virus vaccine is that it can itself cause polio either from the vaccine itself or from the circulation of the vaccine. The circulation of the vaccine can cause small epidemics. There are no such risk from the killed-virus vaccine. In the United States, once polio was eradicated within its population, the Centers for Disease Control decided that the slight advantages of the live-virus vaccine were not worth several vaccine-induced cases of polio each year. The use of the live-virus vaccine was discontinued after 2000, but it is still used elsewhere where polio remains a common threat.

The two vaccines have eliminated polio from most of the countries in the world and reduced early cases from hundreds of thousands per year to only 1000 worldwide in 2001.

Initial attempts to develop a vaccine were hampered by the difficulty of obtaining enough of the virus. Growing it in the brains of monkeys produced only small quantities. Rats were used later, but a breakthrough came in 1948, when John Franklin Enders, Thomas Huckle Weller, and Frederick Chapman Robbins developed a method for growing the virus in the laboratory. This breakthrough greatly facilitated vaccine research. Enders and his colleagues were awarded the Nobel Prize in Physiology or Medicine in 1954.

Simian virus contamination

Some medical researchers have expressed fear that rare human cancers may be linked to the contamination, with the money virus SV40, of a large proportion polio vaccines administered in the 1950s and 1960s. The contamination of the vaccines administered to tens of millions of Americans, in both the injected and oral forms, was first discovered in 1961.

Since then, government officials have insisted there is no evidence the simian virus is harmful to humans. But in recent years, dozens of scientific studies have found the virus in a steadily increasing number of rare brain, bone and lung-related tumors. The simian virus has been detected in tumors removed from people never inoculated with the contaminated vaccine, which has raised further concerns.

Vaccine critics contend that the few studies on the cancer causing effects of SV40 are scientifically flawed, and that health officials have downplayed the potential risks since finding, in 1961, that the contaminant caused tumors in rodents.

See also

• List of vaccine topics
• March of Dimes
• Vaccine controversy

External links

• CGDev.org - 'Vaccines for Development', Center for Global Development
• PBS.org - 'People and Discoveries: Salk Produces Polio Vaccine 1952', PBS
• SFGate.com - 'Rogue virus in the vaccine: Early polio vaccine harbored virus now feared to cause cancer in humans', William Carlsen, San Francisco Chronicle (July 15, 2001)

• Vaccines