The inactivated polio vaccine (or IPV) is now the only vaccine given in the United States to prevent polio. IPV is given as a series of four shots, at 2 months, 4 months, 6 to 18 months, and again at 4 to 6 years of age.
President Franklin Delano Roosevelt, paralyzed by polio, was confined to a wheelchair for most of his adult life. President Roosevelt caught polio when he was in his late 30s. Only about 1 in 10 people who were paralyzed by polio recovered — most were confined to wheelchairs or iron lungs for the rest of their lives.
Polio is caused by a virus and is highly contagious. It affects people differently — some don't feel sick at all, others complain of anything from a sore throat to fever, stomach pain or vomiting, stiff neck or headache. The virus does its damage by first reproducing itself (or replicating) in the intestines, then traveling through the bloodstream where it can infect the brain and spinal cord. Paralysis caused by polio occurs when the virus replicates in and attacks the nervous system.
Two polio vaccines have been available since 1961. The inactivated polio vaccine (IPV) was available first, administered via injection in 1955. A more convenient form, administered as liquid drops via the mouth, was developed in 1961. This oral polio vaccine (OPV) was recommended for use in the U.S. for almost 40 years, from 1963 until 1998. The results have been miraculous: Polio was eliminated from the United States in 1979 and from the Western Hemisphere in 1991.
Since 1998, only IPV is recommended to prevent polio in the United States.
Each vaccine recommended for use against polio had its advantages and disadvantages. The advantage of OPV was that it was almost 100 percent effective at preventing polio. Also, because the vaccine virus was present in the stool, about 25 of 100 people who came in contact with someone who was immunized, would also become immune (this is called contact immunity).
In the early 1960s, when immunization rates in this country were low, contact immunity was an important feature of OPV. However, despite OPV's 40 years of success, there was an extremely rare but frighteningly dangerous side effect: permanent paralysis. Paralysis caused by OPV occurred in about 1 of every 750,000 people after taking the first doses of the vaccine. Since 1979, the time when natural polio was eliminated from the United States, OPV caused about six to eight cases of paralysis each year.
The IPV form, now recommended by the Centers for Disease Control and Prevention (CDC) and the American Academy of Pediatrics (AAP), does not, and cannot cause paralysis. But the original IPV, made in 1955, wasn't a very good vaccine. A few people immunized with at least two doses of the old IPV still caught polio. In the early 1980s, due to advances in protein chemistry and protein purification, a much better inactivated polio vaccine was made. This new IPV vaccine obviated concerns about the old IPV and has been the only polio vaccine recommended for use in the United States since 1998.
OPV was made by weakening the three strains of polio virus that caused disease by growing them in monkey kidney cells. Polio virus that was grown in these cells was so "weakened" that, after it was swallowed, it induced an immune response but didn't cause disease. OPV induced antibodies in the intestines and, therefore, because polio enters the body through the intestines, provided a "first line" of defense against polio. Unfortunately, OPV occasionally reverted back to the natural form, causing paralysis.
IPV, unlike OPV, cannot reproduce itself (or replicate) and, therefore, cannot possibly revert back to natural polio. To make IPV, polio virus is purified and killed with a chemical (formaldehyde). IPV elicits antibodies in the bloodstream, not the intestines. It prevents the virus from traveling through the blood to the brain or spinal cord, thereby preventing paralysis. In a sense, because the vaccine induces antibodies in the bloodstream, and not the intestines, IPV induces a "second line" of defense against infection.
Natural polio has been eliminated from the United States since 1979. However, polio has not been eliminated from the rest of the world. Efforts to eradicate polio have proven challenging. Polio has never been successfully eliminated from Pakistan, Nigeria and Afghanistan. Groups dedicated to eradicating polio continue to work toward elimination; to follow their progress, check www.polioeradication.org.
Since travel makes the world a much smaller place, a recurrence of polio is merely a plane ride away. In countries with high immunization rates, this is less likely, but the potential for spread of polio exists when people are not immunized. Because IPV has no serious side effects, the benefits of the vaccine clearly outweigh its risks.
|Disease Risks||Vaccine Risks|
Plotkin SA, Orenstein W, and Offit PA. Poliovirus vaccine-inactivated and poliovirus vaccine-live in Vaccines, 6th Edition, 2012, 573-645.
Reviewed by: Paul A. Offit, MD
Date: May 2013
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