Until Salk and Sabin produced the first effective polio vaccine in the early 1950's, every hospital had wards filled with survivors of paralytic polio -- all of whom depended on "iron lungs" (a type of mechanical ventilator) to breathe. Since the vaccine came into widespread and routine use, paralytic polio has been almost unheard of in developed countries. In fact, polio has nearly been eradicated, just as smallpox has; unfortunately, there has been a recent resurgence of polio in parts of Africa, mainly in areas where vaccination has been incomplete -- or worse, discouraged by leaders with no appreciation of the risks of not vaccinating children.
The polio viruses (there happen to be three of them) are from a class known as "enteroviruses", because they usually infect the intestines (at least initially). For many people who are infected, a polio infection results in little more than fever, aches and pains, and possibly mild gastroenteritis. In others, though, the virus also attacks nerves that control muscles, which leads to paralysis of those muscles. Before vaccines were introduced, many polio patients became partly or completely wheelchair-bound (Franklin Roosevelt was one famous example). In some cases the virus can attack certain nerves which come directly off the brain rather than through the spinal cord: this can paralyze the muscles of the face and throat, and obstruct breathing. This, combined with paralysis of the chest muscles, often leaves survivors dependent on ventilators.
Before Salk developed the injectable vaccine, polio epidemics were common and usually happened about every 1-2 years. Since the vaccine became widely used, polio has become quite rare in the United States and other developed countries. The Sabin oral polio vaccine actually does a somewhat better job of producing immunity -- for one thing, it stimulates the intestines to produce antibodies, thus protecting against the virus at its point of attack -- and it's a lot easier to give (who really wants shots when you can get a drop on a sugar cube instead?). However, in about 1 out of 2,400,000 doses of the oral vaccine a patient gets paralytic polio from the vaccine itself (caused when the virus mutates into a form that is fully infectious), with 1 case of polio after 750,000 first doses of vaccine (and that includes people catching polio from someone else after receiving the vaccine but before starting to develop immunity). Since 1979, the only cases of polio reported in the United States were associated with the oral polio vaccine. Therefore, the US Centers for Disease Control and Prevention now recommend that we give only the injectable vaccine, and the oral vaccine is no longer available in the United States or Canada. Oral vaccine is still used in countries where polio remains prevalent; the risk of getting polio from the vaccine is much less than the risk of catching polio from someone else in those areas.
The vaccine, whether it is OPV or IPV, is usually given in four doses: at age 2 months, at age 4 months, somewhere between ages 6 and 18 months, and at age 4-6 years. If a child only received 3 doses of vaccine, and the last one was at age 4 years or later, and they were all IPV or all OPV, she does not need a fourth dose. However, if she received some IPV and some OPV, she needs a total of 4 doses for complete protection no matter how old she is now.
The immunity conferred by either vaccine may not be absolutely lifelong,
although the oral vaccine seems to provide longer-lasting protection. Since
polio is still prevalent in developing countries, it's sometimes a good idea for
adults to receive a booster dose of vaccine (usually IPV, since we don't routinely
carry OPV in the United States) before traveling to those areas. Your doctor can
help you decide whether you need a polio booster before a particular trip.
A recent case of polio in Minnesota
At the end of September, 2005, the Minnesota Department of Health reported a case of polio in a 7-month-old girl. The child is Amish, and had received no immunizations; she also has severe combined immune deficiency (SCID), which makes her immune system unable to respond to many infections including poliovirus infection. As of late October, when the US Centers for Disease Control and Prevention reported this case in its weekly report, 32 people living in her community had been tested for polio and 3 people (brothers and sisters of each other living together, but not in the same household as the infected girl) have tested positive. These three people had not been sick and had normal immune systems, but none had been vaccinated against polio.
Analysis of the genes in the virus found in this girl showed that it is a mutation of one of the three polioviruses in OPV. The amount of change between the little girl's virus and the related OPV virus shows that the virus she has has been replicating for about 2 years. Since she is only 7 months old, she must have caught the infection from someone else, and since OPV has not been given to anyone in the United States or Canada since 2000, the virus she has must have started out as OPV given to someone in another country. (Neither she nor any of her family have ever travelled abroad.) Since polio infections most often have no symptoms, or at worse mild illness with fever, most people who might be or have been infected wouldn't even know they had it. The reason we vaccinate against polio is to prevent the unlucky 1 in 200 from becoming paralyzed, or worse.
Remember that IPV, being a killed-virus vaccine, cannot infect a patient. People who have had all of their recommended doses of IPV are not susceptible to polio, even the strain involved in this case. I'm a bit of a libertarian, and I understand and honour the refusal of the Amish to receive vaccinations. And, with SCID, this particular little girl likely would not have become immune to polio even if she had received the vaccine. (Most likely, she will need a bone marrow transplant before she has normal immune function.) On the other hand, the reason for immunizations, whether for polio or for any other disease, is to protect people from those diseases -- especially those people, like this girl, who cannot defend themselves against the vaccine-preventable diseases. If you doubt that vaccination against polio is necessary, go to the CDC Web Site and read about this case. And if your child is not up to date on immunizations, talk to his/her doctor about catching up, and about any concerns you have about the vaccines.