Polio Prevention

In the United States and Canada, polio went from a dread disease to mostly a memory in little more than half a century. This change was largely due to the introduction in the 1950s of polio vaccines, which prevent serious infection from occurring in the first place. Vaccines work by exposing the body’s immune system to a microbial infection that is strong enough to provoke an immune response but not severe enough to result in full-blown illness. Once the body has overcome the vaccine-induced challenge of a given infection, the resulting antibodies can recognize and quickly handle any subsequent invasion by the same agent (see Immunization).

Scientists use two main strategies for producing a vaccine, and both methods have been successfully employed against polio. One strategy uses quantities of virus that have been inactivated, or killed. The first successful vaccine against polio, developed in the United States in the 1950s, employed poliovirus that had been inactivated by the chemical formaldehyde. Once separated from the formaldehyde, the treated virus could no longer produce serious infection, but it retained enough of its molecular character to “teach” the immune system how to recognize and neutralize the virus. Various forms of this inactivated polio vaccine, administered by injection, have been used since the mid-1950s.

The other vaccine strategy uses live virus that scientists grow in a laboratory. Through successive breeding of generations, the cultivated virus is systematically weakened, or attenuated, so that it will no longer cause serious infection. This form of polio vaccine, known as oral polio vaccine, is administered via the mouth by drinking or by being placed in a sugar cube or other food. The first such orally administered vaccines were tested and introduced in the late 1950s. The oral vaccine is easier to administer than the injections used in the inactivated polio vaccine, particularly in remote areas that may lack trained medical staff or proper hospital facilities. Consequently, the oral vaccine eventually almost entirely replaced the injected form.

However, doctors found that using live poliovirus in the oral polio vaccine could involve risk. There is a chance that a dosage may contain improperly weakened virus that is still capable of causing a paralytic infection. In the 1990s the risk of contracting paralytic polio from oral polio vaccine was 1 in approximately 2.4 million doses of vaccine.

This remote, yet real risk posed by live-virus vaccines induced the Centers for Disease Control and Prevention (CDC) to recommend that the schedule of polio vaccines for young children be changed in 2000. Before the change, the schedule recommended that children receive two doses of inactivated polio vaccine via injection, followed by two doses of oral polio vaccine. CDC now recommends that, over the course of their first six years, children receive four injections of inactivated polio vaccine. The Laboratory Centre for Disease Control in Canada makes the same polio immunization recommendations.

In addition to virtually eliminating polio from the United States and the rest of the western hemisphere, polio vaccination has vastly cut the number of cases in developing nations. In 1988, for example, polio cases in Bangladesh, India, and Pakistan totaled more than 25,000. By 1999, vaccination programs had reduced the number of confirmed cases in these countries to about 2,100.