Eradication of poliomyelitis in the United States

Abstract

Mass immunization with oral poliovirus vaccine (OPV) was begun in the United States in 1963, and the least natural outbreak of poliomyelitis occurred in 1972. Since immunization programs fail to reach the total population, eradication has been achieved in the presence of a residual susceptible population of at least 5% (2-5 million children under the age of 15 years). It is proposed that the fade-out of wild polioviruses is explained by their disappearance during the winter, a low point in the yearly cycle of the virus. In the post-eradication era, the continued presence of millions of susceptible children and adults presents a constant potential hazard. Every effort should be made to maintain maximal levels of immunization with oral poliovirus vaccine and to prevent the reintroduction of wild polioviruses into the United States.

PIP: Drawing largely upon recent discussions, this summary addresses 2 main points: how the US succeeded in eradicating poliovirus; and the relative roles of inactivated (IPV) and attenuated (OPV) poliovirus vaccine in eradication. Figure 1, which gives the reported cases of paralytic poliomyelitis in the US over the 1951-77 period, shows that the incidence dropped from 10,000-20,000 cases/year in the early 1950s to a current level of around 10 cases/year. Figure 2, which presents evidence for the 1970-78 period, indicates that cases occurring in the last decade have been either vaccine-associated or imported. There are 2 alternative explanations for the dramatic and largely unpredicted disappearance of poliomyelitis: either all susceptibles have been immunized; or wild virus has been eradicated. National health surveys suggest that 5-10% of the population under age 14 has not received OPV. This does not represent a true estimate of the susceptible population, for OPV spreads to contacts of recipients who may be immunized in this way. Even though the available data fail to provide a precise estimate, they suggest that there is a residual susceptible population in the US that may be conservatively estimated to include at least 5-10% of children under age 14. This observation supports the inference that wild poliovirus has been eradicated in the US and raises another question, i.e., how was poliomyelitis eradicated while retaining a residual population of susceptibles. This question has not been investigated adequately. A major factor in eradication is seasonality. Table 2 presents a hypothetical reconstruction of the preimmunization situation in a population of 1 million. With an estimated 20,000 infections/year, seasonality was such at the trough period only 1/1000 of the total infections for the year would occur in 1 generation period, i.e., 20 infections. Following widespread use of OPV, there were regional fadeouts during trough periods, but unlike the situation with measles, there was no reintroduction of virus into those areas where fadeout had occurred. In sum, a difference in the probability of transmission over long distances may be crucial in explaining the divergent impact of measles and poliomyelitis vaccines. To consider poliovirus vaccines in the context of poliomyelitis eradication, Figure 5 and Table 6 review some of the advantages and disadvantages of the 2 vaccines.