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.
The face of polio
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.
- The disease
What is polio?
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 1955. The inactivated polio vaccine (IPV) was available first, given as a shot in 1955. A more convenient form, given as liquid drops via the mouth, was developed in 1961. This oral polio vaccine (OPV) was recommended for use in the United States for almost 40 years, from 1963 until 2000. The results have been miraculous: Polio was eliminated from the United States in 1979 and from the Western Hemisphere in 1991.
Since 2000, only IPV is recommended to prevent polio in the United States.
- The vaccine
How were the "inactivated" and "oral" poliovirus vaccines made?
OPV was made by weakening the three strains of poliovirus that caused disease by growing them in monkey kidney cells. Poliovirus 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 rarely 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, poliovirus 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.
Why do we use the polio shot and not the oral polio vaccine?
In about 1 of every 2.4 million recipients, the live, weakened virus contained in the oral polio vaccine causes paralysis. The shot does not have this same side effect because, unlike the oral version, the shot contains killed virus that cannot replicate and, therefore, cannot cause paralysis.
Between 1961 and 1996, children in the United States received four doses of the oral vaccine. This changed beginning in 1997 and continued throughout 1999 when children typically received two doses of the shot followed by two doses of the oral version. Since that time, infants have received four doses of the shot.
Many other countries continue to use the oral polio vaccine because it is more economical, easier to administer, and provides better herd immunity; therefore, allowing more people to get the vaccine.
- Relative risks and benefits
Do the benefits of the inactivated polio vaccine (IPV) outweigh its risks?
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.
- Sore throat, fever, stomach pain or vomiting, stiff neck or headache
- Permanent paralysis
- Disease can be fatal
- Pain, redness and swelling at the injection site
Plotkin SA, Orenstein W, and Offit PA. Poliovirus vaccine-inactivated and poliovirus vaccine-live in Vaccines, 6th Edition, 2012, 573-645.