World overview of poliomyelitis: regional patterns and trends.

Reporting of poliomyelitis at national and international levels is often incomplete, and data available are often limited to certain reporting areas or to selected health institutions and therefore do not represent the true extent of the problem. However, despite their deficiencies, the annual data available have provided valuable information of disease trends over the years. Over the past five to six years in every World Health Organization (WHO) region, only a small number of countries accounted for the majority of the cases. These countries show a high endemicity, with periodic peaks of even higher incidence. They comprise, but are not composed exclusively of, the most populous countries in their regions. In addition, a number of countries, usually reporting a small number of annual cases, experience pronounced outbreaks every few years. Poliovirus type 1 predominates in communities with low immunization coverage and a high number of reported cases, and conversely there is a preponderance of types 2 and 3 in well-immunized communities reporting only a few cases. Poliovirus type 1 still causes small outbreaks among the nonimmunized population in well-immunized communities. The data available indicate that if concerted efforts were made to secure and sustain high immunization coverage in a selected number of countries, there would be a disproportionate decrease in the number of cases at regional and global levels.