Academic journal article Bulletin of the World Health Organization

Tetanus Toxoid Coverage as an Indicator of Serological Protection against Neonatal Tetanus. (Research)

Academic journal article Bulletin of the World Health Organization

Tetanus Toxoid Coverage as an Indicator of Serological Protection against Neonatal Tetanus. (Research)

Article excerpt

Introduction

Between 1995 and 1996, a Multiple-Indicator Cluster Survey (MICS) was conducted in more than 60 developing countries to measure progress towards the year 2000 goals of the World Summit for Children and to determine if mid-decade goals had been met (1, 2). The mid-decade and year 2000 goals called for protecting 80% and 90%, respectively, of children against neonatal tetanus, by vaccinating their mothers. The indicator for determining if these goals were achieved is tetanus toxoid (TT) coverage, defined as the proportion of newborns whose mothers, at delivery, were protected against tetanus according to their TT vaccination history (1).

In the 5-dose TT vaccination schedule recommended by the World Health Organization (WHO), minimum intervals between doses and the expected duration of protection after each dose vary by the number of doses received (3). These features make the determination of the TT vaccination status of mothers more complicated than the determination of vaccination status of children, even when exact dates of all TT doses received by mothers are known. Often, however, exact dates of all doses are not known because mothers do not have a record of their TT doses, or because the record they have is incomplete (e.g. doses during different pregnancies may have been recorded in different documents and only the most recent kept). Also, the total number of doses may be difficult for mothers to recall accurately, since some doses may have been received several years in the past. For some mothers, the first doses may have been received in infancy, as a component of diphtheria-pertussis-tetanus (DPT) vaccine.

To understand better the relationship between TT coverage, based on maternal recall, and the level of serological protection achieved against neonatal tetanus, TT coverage and tetanus antitoxin seroprevalence were both measured during a national household survey in Burundi (4). Interviewers asked mothers the number of TT doses they received in each of their previous three pregnancies, if applicable. Despite the potential difficulties for mothers in recalling TT histories accurately, TT coverage (73% (95% CI; 66%-79%)) was close to seroprevalence (67% (95% CI; 59%-76%)). In the MICS, interviewers stopped asking questions about the TT history of mothers once it had been determined that a mother was protected at the time of her last delivery (1); if a mother stated she received two vaccine doses during her last pregnancy, no questions were asked about previous doses. Also, the simplifying assumption was made that all doses met the minimum interval requirements.

In 1995, the WHO Steering Committee on Epidemiology and Field Research recommended that tetanus serology be added to the MICS in 2-3 countries to determine the accuracy with which TT coverage from the MICS indicated the prevalence of serological protection (5). Our study was conducted in response to this recommendation, and to our knowledge was the only such study completed at mid-decade.

Methods

Sampling

In the CAR MICS, conducted from January to March 1996, interviewers returned to the same 231 clusters that were used in the 1994-95 Demographic and Health Survey (DHS). For the DHS, the country was divided into 11 strata: an urban and a rural stratum in each of the five regions, and Bangui, the capital (6). The primary sampling units (PSUs) were census tracts, selected with a probability of selection proportional to the total population as determined by the 1988 national census. Households in each selected PSU were mapped and listed. For the MICS, 36 households from the complete DHS household list for each PSU were selected by systematic sampling. In each household, MICS questionnaires were used to obtain information on all children under 16 years of age and their mothers, including a TT questionnaire for the mothers of children younger than one year of age.

The survey team member responsible for obtaining filter-paper blood specimens visited one-quarter of the households in each cluster chosen by systematic sampling, and asked all mothers of children younger than one year of age for consent to obtain a filter-paper blood specimen for tetanus serology. …

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