Poliomyelitis: present epidemiological situation and vaccination problems.

In 1988 the World Health Organization (WHO) declared its commitment to the goal of global eradication of poliomyelitis by the year 2000, generating new enthusiasm in pursuing this objective worldwide. The nations involved are supported by a coalition of partners, including Rotary International, Centers for Disease Control and Prevention of Atlanta, Ga., UNICEF, and the WHO itself. Interventions in countries in which poliomyelitis is still endemic are also supported by the authorities of numerous countries free from this disease. According to WHO data (29), 35,251 cases of poliomyelitis were reported around the world in 1988, 8,635 in 1994, and 6,197 in 1995, the last year for which complete data are available. The last figure demonstrates an 82% decrease of the number of cases since 1988. In 1994 the International Commission of Certification of Eradication of Poliomyelitis declared the Americas polio free (30), and in 1995 no case was reported in 150 countries (29). The vast majority of the cases reported were in developing countries in which only one of the paralytic forms that occur in reality. A WHO estimate put the true number of new cases of paralytic poliomyelitis at 80,000 in 1995. The incidence rates in east Mediterranean countries are among the highest, and in 1995 12% of all the cases reported worldwide were in these countries. Before 1964, when the oral Sabin polio vaccine (OPV) became available, there were on average 3,000 cases of paralytic poliomyelitis in Italy each year (26, 27), with a mortality rate of around 10% (25). The permanent sequelae of this disorder account for an important number of paralyzed subjects in the present Italian population. Immediately after the introduction of the oral vaccine, the incidence of this terrible disease fell drastically, so the number of cases reported in the last decade can be counted on one hand (4, 16, 17). Despite this, poliomyelitis, unlike smallpox, still has not acquired the status of a historical viral disease. In recent years, great alarm has been generated by outbreaks of paralytic poliomyelitis in vaccinated populations in which the levels of immunity against poliovirus are not adequate or not controlled. For example, epidemics were observed in Finland in 1984, Senegal and Brazil in 1986, and Israel and Oman in 1988, all countries in which vaccination is widely deployed. Four epidemics were reported between 1991 and 1992. The first, in 1991, was in Bulgaria, which uses oral vaccination. Forty-three subjects developed paralytic type 1 polio; 88% of them belonged to a nomad community and had not completed or even started a vaccination schedule (31). The second epidemic occurred in The Netherlands, where inactivated polio vaccine (IPV) is used, and involved 68 patients with type 3 poliovirus, members of the Amish community which refuses vaccination and which in the past (1978–1979) had already figured in a similar outbreak of polio type 1 (11, 32). The third epidemic was in Jordan, where in the winter of 1991–1992 flaccid paralysis was observed in 55 patients and confirmed as acute poliomyelitis type 1 in 32 (56%). All 55 were under 5 years of age, and half of them were still not immunized, although the health authorities estimated that 95% of children of that age had received at least two doses of vaccine following a national vaccination day (33). Poliovirus had presumably been imported by the numerous refugees arriving in Jordan from the area involved in the Gulf War. Lastly, in Malaysia—where OPV is used, vaccination coverage is over 90%, and no cases of poliomyelitis had been reported since 1985—three cases of paralytic poliomyelitis occurred in 1992 in a group of religious fundamentalists who refused any form of vaccination (34). A series of seroepidemiological investigations have been performed, particularly in the countries where epidemics have occurred, to check the immune titers of the populations involved. The serological data obtained in different series indicate a gap in immunity against polioviruses, especially type 3 (10, 18, 28, 35). One particularly important finding was the wide antigenic variations detected in the wild poliovirus strains isolated in Israel and Finland with respect to the strains used in vaccines. Other methods useful in monitoring the epidemiological situation, although less reliable than serology, consist of surveys of subclinical infections and detection of polioviruses in the environment. The presence of subclinical infections is determined by testing stools for the virus; this is not an easy investigation, because of the need for cell cultures and the practical difficulty of checking a significant number of samples. The last cases recorded in Europe, apart from those in Bulgaria in 1991 and the Netherlands in 1992–1993, were epidemics in the Russian Federation in 1995 and Albania in 1996. The vaccination rate has fallen in both countries, for obvious political and economic reasons (36). As already mentioned, in Italy the situation is excellent as regards paralytic poliomyelitis. For this reason and also because of the arrival of devastating new viral diseases, epidemiological research on polioviruses generates little interest and few or no funds. However, as wild polioviruses continue to circulate in populations with which our countries maintain numerous links (business, tourism, immigration), and because the recent epidemics suggest that it is better not to rest on one’s laurels, the Milan University Institute of Virology decided to undertake new investigations in the field of polioviruses, with the partial support of the Lombardy region health authorities. A serological study (7) on adolescents and young adults who had completed a regular poliomyelitis vaccination schedule demonstrated that 10 to 15 years after the end of the schedule, the antibody titers could be considered good for poliovirus types 1 and 2 but not poliovirus type 3; antibodies to type 3 were not detected in 13 to 20% of the subjects examined (7). The problem of subjects who are seronegative years after vaccination is interesting and important. Two hypotheses can be formulated: a lack of take of the vaccine strains or a decline of * Corresponding author. Mailing address: Istituto di Virologia, Università di Milano, via Pascal 38, 20133 Milano, Italy. Phone: 39 2 2367494. Fax: 39 2 26680713. E-mail: [email protected].