The goal of eradication of poliomyelitis has been complicated by some events in 2000–2001. First, the discovery of circulation of a vaccine virus strain that mutated to regain its disease-causing ability in Hispaniola. This outbreak in the Dominican Republic and Haiti was the first in the Americas since 1991 and occurred in areas of very low vaccination coverage . Second, the polio cases in Bulgaria. There had been no reported cases of polio in Europa since November 1998. In spring 2001 two gypsy unvaccinated children were paralyzed by a wild poliovirus in Bulgaria, which was imported from India . These outbreaks send a warning to all countries to maintain their guard against polio, despite the rapid decline in its incidence. Sufficient surveillance systems and high levels of population immunity can prevent polio outbreaks.
The level of immunity to poliomyelitis has been checked in Germany at regular intervals.
For the first time in Germany the results were expressed in International Units. A representative serum panel from the German Health Survey was investigated. Additionally, sera from children less than 18 years old were included in this study. These sera were not representative for all selection criteria because they originated from diagnostic laboratories. A representative study of the health status of persons under an age of 18 years is prepared by the Robert Koch Institute. The pilot phase has started in 2001. Within the next years valid data will be obtained for the younger population of Germany.
The given threshold values of 0.075 lU/ml, 0.180 IU/ml, and 0.080 IU/ml were calculated for Polio 1,2,3 respectively. The calculation was based on the assumption that the new criteria (threshold in IU/ml) should give comparable results with respect to the old one (threshold of serum dilution).
Overall, neutralizing antibodies against poliovirus type 1, 2 and 3 were detected in 96.2%, 96.8% and 89.6% of samples, respectively. This seroprevalence indicates a very high level of immunity of the German population. Although the prevalence of antibodies against poliovirus was generally somewhat lower for type 3 than for types 1 and 2, it was still close to 90%.
Data presented in this study are concordant to those obtained in other countries [8, 9].
A previous seroepidemiological study in Germany as well as this one revealed substantial regional variations of immunity levels to Polio 3 . Compared to the results of this study other German studies have shown remarkable lower antibody prevalence to Polio 3 among blood donors in Berlin and in hospitalized patients [11, 12].
Limitations in some of these serological studies due to non-representative samples or methodological reasons resulted in different conclusions regarding immunity status. Standardization of the neutralization test as well as expressing results of serological studies in International Units will permit better comparability of polio immune status of populations in different studies, and in different countries.
Immunity to poliomyelitis is largely dependent on humoral neutralizing antibodies, both after natural infection and after vaccination. It is unclear at present whether all those persons with a low level or no detectable antibodies are susceptible to infection; some, particularly the elderly, may be protected by memory immunity, an accelerated antibody response because the immune system has been primed previously. It has also been shown that enterovirus infections induce T-cell immunity . The production of local secretory IgA antibodies in the gut mucosa may play a major role in protection [14, 15].
Although IPV offers excellent protection against disease, it is less effective in preventing poliovirus infection because of limited mucosal immunity. IPV recipients may contribute to the circulation of outbreak virus. Extensive circulation of Polio 3 was observed during 1984 outbreak in Finland. A study from the Netherlands has also shown that poliovirus circulation occurred during the early phase of the 1992–93 poliomyelitis outbreak. Most infected children had not been vaccinated . A high level of immunity of the German population, including immigrants, must be monitored after the switch from attenuated to inactivated polio vaccine (OPV to IPV).