At the beginning of the 20th century, infants were often protected from the polio virus by their mothers' antibodies, which were still circulating in the blood.
Babies often experienced only mild symptoms, but severe polio cases began to appear in older children who did not have the same immunity as infants. Polio does not have a cure, so when vaccines were developed, they truly changed the course of history.
There were many major polio outbreaks in the late 1800s and early 1900s. A 1916 outbreak in New York City killed more than 2,000 people, and a country-wide outbreak in 1952 killed more than 3,000 people.
Those infected during these outbreaks who survived the disease faced long-term complications. Because there is no cure, the need for a vaccine was urgent.
Most people with polio have minimal symptoms, including fever, tiredness, nausea, stomach pain, headache, or sore throat. Some people do not have any symptoms at all. A small group of people, though, will experience severe symptoms involving the brain and spinal cord, including meningitis and paralysis.
Paralysis caused by polio can improve, but some people develop muscle weakness and pain as many as 40 years after infection.
The CDC recommends that children in the U.S. get four doses of the injectable vaccine, given at two months, four months, sometime between six and 18 months, and sometime between age four and six.
People who plan to travel internationally should ensure that their and their children's vaccines are up to date. Any adult who is not fully vaccinated or at increased risk of exposure should consider getting vaccinated. A single lifetime booster is also available for fully vaccinated adults with an increased risk of exposure.
Only a few people should not get the polio vaccine, including anyone with life-threatening allergies. Anyone who has ever reacted to a polio vaccine should not get another dose.
People who are not feeling well or have a moderate or severe illness can wait until they feel better to get the vaccine.
In 1954, Dr. John Enders, Dr. Thomas Weller, and Dr. Frederick Robbins won the Novel Prize in Physiology or Medicine for discovering how to grow the polio virus in various tissue cultures in a laboratory setting. This important discovery paved the way for the research required to develop polio vaccines.
Dr. Jonas Salk was the lead researcher on the team that discovered the first successful polio vaccine. Salk used the dead polio virus in his vaccine, believing it could give people immunity without infecting them, which was radical thinking at the time. He developed a way to grow a large amount of the polio virus and then killed them with formaldehyde.
The vaccine was first successfully tested in monkeys and then humans, starting with children who already had polio. Blood tests showed that their antibodies increased significantly after vaccination, and a sizeable controlled field trial began. On April 12, 1955, the director of the trial announced that the vaccine was 90 percent effective.
Salk's vaccine was licensed on the same day the results were released. Two years later, in 1957, annual polio cases dropped sharply from 58,000 to 5,600; by 1961, only 161 cases were reported.
Salk believed his vaccine should be easily accessible and knew that the only way to eliminate polio was to ensure everyone could receive a vaccination. Six pharmaceutical companies produced Salk's vaccine, and he did not profit from sharing the process.
Dr. Albert Sabin developed an oral polio vaccine around the same time Salk was working on his killed-virus vaccine. Sabin believed that an oral vaccine would be easier to administer, and he used a live virus to develop it.
He grew and tested many strains of the polio virus, eventually finding three mutant strains that stimulated antibody production without causing paralysis.
Testing Sabin's oral polio vaccine was difficult because Salk's vaccine was already being used in the U.S. with impressive results.
In 1957, Sabin conducted successful field studies in the Soviet Union; in 1960, the trial's success led to approval for use in the U.S., and in 1961, American production of the oral vaccine began. Pfizer mass-produced Sabin's vaccine, and it replaced Salk's injectable vaccine in many parts of the world.
There was some debate about which vaccine was better. Although there was some evidence that the live vaccine could cause paralytic polio in rare cases, Sabin refused to believe it. Salk believed that his killed-virus vaccine was just as effective as the live-virus vaccine, without any risks of paralysis.
In 1999, the U.S. returned to using only Salk's killed-virus vaccine because of the slight risk that the live, oral vaccine could cause polio.
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