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Introduction: Italy and Europe were declared polio-free in June 2002, but increasing migration, even from undeveloped countries where polio still exists, may lead to a come-back of circulating poliovirus (PV) and infection in previously polio-free areas. It is consequently advisable to continue to monitor the immunity of the population in polio-free areas. The aim of this study was to assess the current prevalence of neutralizing antibodies in recently-immunized children and in elderly people who were never vaccinated., Methods: Sera from 511 healthy subjects resident in the Veneto Region were examined to assay their antibody titer for PV 1, 2 and 3, using the microneutralization test. Data were analyzed by chi-squared test, Student's t-test and linear regression analysis, using EPI-Info 2000 supplied by the Centers for Disease Control and Prevention (Atlanta, GA, USA)., Results: Neutralizing antibodies in group A (231 subjects aged 1-17 years) showed significantly higher geometric mean titers (GMTs) than in group B (280 subjects aged 65-100 years) for all three PV serotypes (P < 0.001). Nobody simultaneously lacked neutralizing antibodies for all three serotypes. There were no difference between the two groups in terms of male/female GMTs for the three PV Antibodies decreased with time since vaccination, but the difference was only significant for PV 3. GMTs were lower in the elderly, with no significant difference among the three PV., Discussion: The population examined showed a good level of protection against the three PV strains: both groups A (vaccinated) and B (naturally immunized) revealed a valid immunity to poliovirus., Conclusion: Immunization programs and immunity status population screening are still advisable until polio has been world-wide eradicated.

Widespread use of poliovirus vaccines has lead to a dramatic decline on the incidence of paralytic poliomyelitis in the Western Hemisphere. In Italy, the use of live attenuated vaccine (OPV) has lead to virtual eradication of poliomyelitis. However, wild poliovirus is still endemic in some less developed countries, and there is still a risk of importation of wild-virus poliomyelitis as result of immigration from these areas. Therefore, a serologic study of the immunologic status to poliovirus was carried out in 242 immigrants from less developed countries in the Veneto Region (North-East Italy). The overall prevalence of serum neutralizing antibodies was 98.3 percent for poliovirus type 1, 99.6 percent for type 2, and 95.9 percent for type 3. The modal titres were 1:128 (21.1 percent), 1:512 (26.4 percent), and 1:32 (22.3 percent) for poliovirus type 1, type 2, and type 3, respectively. No subject was simultaneously lacking neutralizing antibodies to all three polioviruses. The mean duration of stay in Italy before testing was 1.53 years (range 1 month to 7 years). These results show a good level of the immunity status, similar to those found in the Veneto general population. Although the risk of paralytic poliomyelitis in our population is very low, seronegative immigrants originating from areas of high endemicity could reintroduce wild poliovirus. It is therefore important to maintain a careful surveillance system on the importation of wild poliovirus and to plan vaccination programs for unprotected immigrants.

In 1976, a cohort survey on the persistence of neutralizing antibodies in children regularly immunized 1 to 4 years earlier with the Sabin attenuated vaccine (OPV) was undertaken in the Venice mainland. Subsequent serological evaluations in the same cohort were carried out in 1983 and 1993. A macroneutralization test using a 1:4 initial serum dilution was utilized in the 1976 and 1983 survey years. In the 1993 survey a microneutralization test using a 1:2 initial dilution was utilized. In this survey, however, sera were tested using both the latter microneutralization test and the former test. Using the former method, the results indicate that the OPV-induced humoral immunity to poliovirus 1 and 2 remain fairly stable after the initial decrease, whereas antibodies to poliovirus 3 are further declining. Using the latter more sensitive method the seropositivity rates were found to be equal or close to 100 percent. The results of our follow-up survey thus indicate that the OPV-induced humoral immunity is long-lasting when tested with a highly sensitive and reproducible method. The clinical protection that ensues after OPV-immunization is probably lifelong similar to that which follows the natural infection.

The immunity state to poliovirus types 1, 2, and 3 of a population aged 2 to 75 years was determined by examining 274 sera collected in the Venice mainland (North-East Italy). Altogether, the neutralizing antibody prevalences (at a titre > or = 1:2) for poliovirus 1, 2, and 3 were 99.0 percent (geometrical mean titres [GMT]: 72.1), 99.6 percent (GMT: 95.9) and 98.2 percent (GMT: 17.3), respectively, and all the age groups also showed very good levels of humoral immunity. High antibody titres (> or = 1:256) to one or more types of poliovirus were demonstrated in older age groups also, possibly indicating exposure to natural polioviruses or contact with vaccine strains. Although probably all the older subjects had practically acquired their antibodies as a result of natural infection and those under 30 through vaccination, these results indicate that the humoral immunity against poliomyelitis in our population is satisfactory, and the maintenance of such good protection level depends on an effective immunization program.