Current status in Japan
    In Japan, 1,500~3,000 cases of poliomyelitis were reported each year in the 1950s. A large scale outbreak striking more than 5,600 people occurred primarily in Hokkaido in 1960. Then during the following year, in 1961, an even greater number of patients were observed in the region of Kyushu from the beginning of spring. Since there were visible signs of a large epidemic, the system of epidemic prevention using the inactivated polio vaccine (IPV) was switched to collective administration of the oral attenuated live polio vaccine (OPV) in order to block this large outbreak. The emergency collective immunization all over Japan of children under the age of five years that was kicked off in July 1961 produced a remarkable effect. Indeed the outbreak was stopped spectacularly. The number of patients decreased in 1962 to 1/10 over the previous year. There were less than 100 cases in 1964, and from 1970 onward, the number of cases did not exceed 10. As regards the wild poliovirus, the type 3 was isolated in 1971 and the type 1 in 1980. However, no wild polio virus was isolated subsequently. Therefore we may consider that the wild poliovirus was eradicated in Japan in the early 1980s.
Current situation in the world
    Brazil implemented a nationwide collective immunization campaign in 1980 following Japan's example, partly under Dr. Sabin's recommendation, and the country remarkably succeeded in the control of polio.
    In consideration of these results, the WHO declared during its general meeting in 1988 that polio should be eradicated globally by the year 2000. Although this objective could not be reached in that year, a remarkable progress toward this goal was achieved with the introduction of the system of national immunization days (NIDs) in the countries where the wild poliovirus was prevalent in the 1990s. The number of countries where this disease is endemic was 125 in 1988. It decreased to six in 2003 (Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan). The number of reported cases fell from approximately 350,000 in 1988 to 682 in 2003, and 95% of them were found in three countries only, Nigeria, India and Pakistan 1).
Advantages of OPV and WHO's polio eradication strategy
    Since the OPV provides immunity through the natural infection route, it is useful for the protection against infection, not only because of the induction of humoral antibody, but also through local intestinal immunity. Besides, since a large amount of virus is shed over several weeks from the children who received the vaccine, it also infects children who have not been vaccinated and can thus provide them with immunity. This is an advantage of OPV and it is the most effective for blocking the transmission of poliovirus. Other merits of the vaccine are its cheap price and its convenience of use since it can be administered via the oral route. Since humans are the only hosts for the poliovirus, the vaccine displays a remarkable efficacy when it is administered at one time in a region. Thus, the number of polio-sensitive individuals decreases in that region and the wild poliovirus spreading in the region is exterminated. Nonetheless this also leads to disadvantages for OPV; the appearance of vaccine-associated paralytic poliomyelitis (VAPP) caused by vaccine-derived polioviruses (VDPVs), as will be mentioned later.
    The WHO has implemented its eradication strategy along the following four lines. 1) Implementation of NID or SNID (sub-national immunization day) campaigns in endemic regions or in countries where the endogenous wild poliovirus is present, and continuation of routine vaccination and maintenance of high vaccination coverage in non-endemic countries. 2) Depending upon the regions, visits to individual homes and thorough vaccination (introduction of the mop-up system). 3) Survey of patients with acute flaccid paralysis (AFP), sampling of the feces from AFP patients, virus isolation, and if poliovirus is found, differentiation between the wild poliovirus and VDPV, and thorough vaccination of children in their entourage. 4) Upgrading of laboratories able to perform the above-mentioned tests and establishment of a network of research facilities capable of differential diagnosis.
    Remarkable results were successfully obtained by these strategies.
Problems with OPV
    VAPP that strikes children taking the OPV is inevitable, although the incidence is about one case per 2~3 million subjects receiving OPV. Besides susceptible individuals may be infected by the virus shed in the environment and contract the disease. In Japan cases of parents who contracted polio from children receiving the vaccine are rare. Nevertheless VDPV may circulate in the environment. The number of susceptible subjects increases due to the fall in the vaccination rate. In addition the infection is transmitted from person to person in those regions where the public health environment is poor and where contacts between children may be frequent. VAPP cases have been reported in Dominica, Haiti 3), Egypt 4), Madagascar and other countries. In fact although the wild poliovirus has been exterminated, there is an increasing concern about the risk of poliomyelitis caused by circulating VDPV(cVDPV) in the environment.
Future strategy
    The WHO began a study for actions as the final stage of polio eradication from around 1998 under the name "end game". It was well understood that it would suffice to use IPV for the eradication of the cVDPV. However, although this poses no problem in economically affluent industrialized countries, switching from OPV to IPV in developing countries was considered to be extremely difficult for several reasons, such as the higher price for IPV injection compared to OPV, the cost for having on hand personnel able to inject the vaccine, etc. Furthermore, VAPP caused by the cVDPV became evident at the beginning of the year 2000, and problems of cVDPV would never be solved as long as OPV is used. Therefore, a difficult decision had to be taken on how long the administration of OPV should be continued. Recently the WHO launched eventually the following policy.
    The WHO announced in January 2004 a polio eradication strategic plan for 2004~2008 2). This plan calls for the following activities. The surveillance activities will be strengthened and upgraded in 2004`2005, vaccination with OPV will be thoroughly implemented in the remaining regions where the polio is still endemic, and the transmission of wild poliovirus will be interrupted. In 2006~2008, the eradication of polio will be certified, the manufacture of IPV including the inactivated Sabin strain vaccine developed in Japan will receive approval in preparation for the cessation of OPV administration all over the world from 2009 onward, and laboratory containment of infectious wild poliovirus and potentially infectious materials will be completed.
    As one may understand from these explanations, people traveling to countries where polio is still present and in neighboring countries are advised to receive a booster of polio vaccine. It is also earnestly hoped that immunization will be carried out with IPV manufactured with the Sabin strain, which is expected to be developed in Japan too as soon as possible and on the early commercial application of which the WHO is also pinning its hopes. However, when switching to IPV, care will have to be taken to prevent the vaccination rate from dropping.