The Topic of This Month Vol.18 No.1(No.203)
Poliomyelitis has been a notifiable disease since 1947 in Japan and thereafter the cases have been grasped in the nation-wide scale. From around 1949, polio epidemics were reported from various parts of the whole country. In 1960, a large-scale epidemic broke out in Hokkaido and polio patients totaled more than 5,000 in one year. In 1961, emergency national immunization with imported live vaccine was carried out. In 1963, regular vaccination was started, resulting in abrupt decline in polio patients, and poliomyelitis has so far been brought under almost complete control (Fig. 1). Mass vaccination with live oral poliovirus vaccine (OPV) has been carried out in spring and autumn (two doses for each infant).
Since 1962 the National Epidemiological Surveillance of Vaccine-Preventable Diseases (NESVPD) has been implemented to monitor the possibility of poliomyelitis occurrence in populations on the basis of the virus isolation and seroepidemiology conducted in cooperation with prefectural and municipal public health institutes. The former is to carry out virus isolation from stool specimens of healthy children to monitor the circulation of poliovirus at nonvaccination periods. Besides, typical paralytic polio patients have been subjected to virological and serological confirmation. The isolated polioviruses are subjected to laboratory testing as to whether they are vaccine-related or wild-type strains. The seroepidemiologic survey continuously pursues general healthy individuals for poliovirus-neutralizing-antibody prevalence to monitor the efficacies of vaccination and the population immune status. The following are the summaries of the results obtained through NESVPD during the period from 1962 through 1995:
1) Virus isolation: In attempts to isolate viruses from healthy children (about 76,000 specimens), wild-type poliovirus was never isolated.
The results of surveillance of typical paralytic polio patients are shown in Table 1. Of 120 virus isolates from 95 patients during the 34 years, only three were wild-type viruses, two type 1 and one type 3; all the others were vaccine-related strains that had undergone reversion in the neurovirulence. After 1991, three unvaccinated paralysis patients (contact cases) were reported (see IASR Vol.14,No.12 and Vol.15,No.1). Isolation of wild-type poliovirus from other patients than typical paralytic polio ones was reported in one case of type 1 virus in Aichi Prefecture in 1984 (from a pharyngeal swab from a 7-year-old girl with encephalitis) (see IASR Vol.6,No.1) and another case of type 3 in Shiga Prefecture in 1993 (from a pharyngeal swab from a 13-year-old boy with inflammation of the upper respiratory tract) (see IASR Vol. 14, No. 11).
2) Seroepidemiology: The survey performed in 1961, before the OPV vaccination, revealed a neutralizing-antibody prevalence pattern which is characteristic of natural infection, indicating increase in the antibody positives to any type as the subjects grow older. In 1963, after the emergency vaccination, high antibody-positive rates were obtained with all age groups. Figure 2 shows the trend of the antibody-positives (with neutralizing antibody titer 1:4=<) during 1972-94. In the survey performed in 1972, the antibody-acquiring rate after vaccination (antibody-positive rate of the age group of 1-2 years) was satisfactorily high with respect to type 1 and 2 viruses, but low with respect to type 3. The survey performed in 1978 revealed an improvement in the antibody-acquiring rate against type 3 virus, whereas that against type 1 became low. The survey performed in 1988 found an improvement in the rate against type 1 virus. The population with a low type 3 antibody-acquiring rate detected by the 1972 survey and those with a low type 1 antibody-acquiring rate by the 1978 survey showed still low antibody-positive rate even in the 1994 survey when they had grown older (shown with big circles in Fig. 2); this indicates that now people in Japan acquire poliovirus-neutralizing antibody only by vaccination. As for the type 2 antibody prevalence, all age groups showed high antibody-positive rates in any survey year.
To clarify the antibody-positive rates in the specified age groups, the results of serosurveys on serum specimens collected from a total of 7,306 subjects during a 4-year period from 1991 through 1994 have been summarized (Fig. 3). The age of every subject was adjusted to that in 1994, and the antibody-positive rate of those with a titer 1:4<= in every age group older than 5 years was depicted. The rate of antibody positives to type 1 virus of the 18-year group is as low as about 50%; that to type 3 virus of the 22-year group is also about 50%.
Type 1 virus is often isolated particularly from polio patients living in polio epidemic areas. The Ministry of Health and Welfare recommended that the age groups with low rate of antibody-positives to type 1 virus (corresponding to those who were born in 1975-1977) receive a booster dose of vaccination when going out to countries where wild-type polioviruses still exist (see p. 3 of this issue).
The vaccination coverage has been kept at a rate higher than 90% since 1981. The antibody-acquiring rate with the current vaccine is almost 100% with respect to types 1 and 2 and 90% with respect to type 3 viruses. Immunogenicity of OPV and its reversion potential in the neurovirulence (paralysis) are to be monitored continuously.