Using the Two-Source Capture-Recapture Method to Estimate the Incidence of Acute Flaccid Paralysis in Victoria, Australia. (Research)

Article excerpt

Voir page 850 le resume en francais. En la pagina 850 figura un resumen en espanol.

Introduction

In 1988 the World Health Assembly launched an initiative aimed at eradicating poliomyelitis by 2000. The strategy subsequently adopted by WHO relies on immunization and surveillance. Countries where the disease is endemic seek to interrupt transmission by using oral poliovirus vaccine in a combination of high routine immunization coverage of infants and supplementary immunization of children under 5 years of age, delivered through national and subnational immunization days. Mopping-up campaigns may also be necessary. Countries where poliomyelitis is not endemic rely exclusively on high routine immunization coverage of infants.

All countries are encouraged to conduct surveillance for cases of acute flaccid paralysis (AFP). AFP surveillance is a sensitive marker of poliovirus circulation and for children under 15 years of age, is the recommended method for the detection of poliomyelitis caused by wild poliovirus. WHO recommends that surveillance systems should be sufficiently sensitive to identify an annual rate of non-poliomyelitis AFP of 1 per 100 000 children aged under 15 years. Each case should be reported and investigated within 48 hours of identification (1).

AFP is a working classification used to describe patients presenting with acute onset of paralysis in one or more limbs or acute onset of bulbar paralysis. There are many possible causes, but Australian data suggest that 63-72% of cases are attributable to either Guillain--Barre syndrome or transverse myelitis (2). Thorough clinical investigation, adequate virological examination of stool specimens and follow-up at 60 days after onset are required in order to determine the cause of paralysis.

In Australia, active AFP surveillance was introduced in 1995 as a joint initiative of the Australian Paediatric Surveillance Unit and the Commonwealth Department of Health (2). This surveillance relies on a network of paediatricians who voluntarily notify cases at the time of presentation. Paediatricians also make monthly reports of rare conditions seen, including cases of AFP. The notified cases are entered on a national register.

In 1995-99, Australia did not attain the recommended target rate for AFP notifications of 1 per 100 000 children aged under 15 years. In order to satisfy WHO requirements for poliomyelitis-free certification in Australia, retrospective hospital record reviews were conducted in three states and one territory of the country with a view to identifying additional cases (3). Australia and, indeed, the entire Western Pacific Region, were certified free of poliomyelitis in 2000 (4).

A previous study in the State of Victoria, Australia, in which the two-source capture--recapture method was used, indicated that the average annual incidence of AFP was about 1.7 per 100 000 among children aged under 15 years in 1995-97 (5). The current retrospective study was also conducted in Victoria, where the projected population was 4 707 590 in 1999, of whom 948 124 were estimated to be aged under 15 years (6). We aimed to assess the completeness of AFP ascertainment in Victoria during 1998-2000, to estimate the incidence of the condition, and to determine its common causes among children aged under 15 years.

Methods

The two-source capture--recapture method was employed to estimate the incidence of AFP cases and to evaluate case ascertainment with respect to data in the national register (7). Estimation methods based on capture--recapture arose from census studies on animal populations. They have been employed in epidemiological studies on the incidence and/or prevalence of given diseases or injuries as well as in evaluating the completeness of case ascertainment (8). Such methods make it possible to compare two or more independent lists of cases in order to estimate the total number of cases in a given population (9). …

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