Polio eradication – the validity of surveillance indicators

A major pre-requisite for polio-free certification by the World Health Organization is that the local surveillance system successfully detects one case of non-polio acute flaccid paralysis (AFP) per 100,000 children below 15 years of age per annum and that no cases of polio occur for three consecutive years. Mpumalanga, a rural province in the northeast of South Africa, implemented an enhanced surveillance system, which consisted of training hospital ICNs to rapidly report and correctly respond to nine infectious disease syndromes, including AFP. Weekly zero reporting is a component of the system. The non-polio AFP reporting rate per 100,000 children below the age of 15 years increased from 0.56 in 1997 to 0.91 in 1998 after introduction of the enhanced surveillance system, with more than 80% of the units reporting weekly. All units reported weekly from April 1999 to December 2001. Although non-polio AFP reporting rates were 0.27 (1999), 1.18 (2000) and 0.87 (2001), the 95% binomial exact confidence intervals for all years included 1 per 100,000. A review of paediatric admissions from January 1998 to December 2001 at all hospitals revealed that only five AFP cases had been missed by the enhanced surveillance system. The low international AFP reference rate and attendant variation expected due to chance, particularly in areas with relatively small populations, is an important factor that deserves more attention as we approach global polio eradication.