Drugs & Medication

Two polio vaccines are used throughout the world to combat polio. The first was developed by Jonas Salk, 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 oral vaccine was developed, it gained 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 the potential for spreading the wild virus. Third, the live-virus oral 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 immunization 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 result in sporadic cases of polio either from the vaccine itself or from the circulation of the vaccine. There is neither risk from the killed-virus vaccine.

In each country, as the incidence of wild polio reaches a very low level, the time comes to change from live vaccine to killed. In the United States the Centers for Disease Control decided this point was reached in 2000 and use of the live-virus vaccine was discontinued. In the UK the change occurred in 2005 and for convenience polio vaccine was combined with Tetanus toxoid and Diphtheria. In states with higher incidence and thus a different relative risk between efficacy and reversion, live vaccine is still used. The live virus also has stringent requirements for transport and storage, which are a problem in some areas.

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 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 contamination by the monkey virus SV40 of a proportion of polio vaccines administered in the 1950s and 1960s. 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 assert 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.

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)