Academic journal article Bulletin of the World Health Organization

Predicting and Comparing Long-Term Measles Antibody Profiles of Different Immunization Policies

Academic journal article Bulletin of the World Health Organization

Predicting and Comparing Long-Term Measles Antibody Profiles of Different Immunization Policies

Article excerpt

Voir page 622 le resume en francais. En la pagina 622 figura un resumen en espanol.

Introduction

Before the introduction of the mass immunization programme in 1978, measles was endemic with a 2-year epidemic cycle in Taiwan, China (1). The most recent outbreak affecting the whole island occurred in 1988-89 (1-2). After that outbreak, a goal for measles elimination by 2000 was established in 1991. Towards achieving this, an MMR (measles, mumps and rubella combined vaccine) campaign targeting primary and secondary schoolchildren (6-15 years old) was conducted in 1991-94 and reached 90% of the target population (3). Meanwhile, a 2-dose policy with 1 dose of measles vaccine at 9 months of age and 1 dose of MMR at 15 months of age has been implemented since 1991. Based on the vaccine coverage survey conducted among children aged 13-24 months in 1993-94, the mean coverage was 84% (range: 77-96%) for one dose of measles vaccine and 69% (range: 60-83%) for one dose of MMR vaccine (3). Recent health statistics showed that the MMR coverage in 2-year-old children in 1995 was about 80-85% (4). In addition, immunization requirements have been implemented by screening vaccination records and immunizing unvaccinated individuals among newcomers (6 years of age) to primary school since 1991 (`mop-up'). This policy has been extended to kindergarten since 1994. Under the strategy of `mop-up', the overall vaccine coverage with at least 1 dose of MMR vaccine in primary schoolchildren was 96% in 1995 (4).

In addition to improving measles vaccine coverage, since 1992 measles surveillance has also been strengthened by incorporating active case investigation and laboratory diagnosis. Except for a small-scale outbreak with 33 confirmed cases in the northern part of the island in 1994, only sporadic confirmed cases were identified in 1992 (7 cases) and 1993 (2 cases). No confirmed cases were detected in 1995 and 1996. However, 7 confirmed cases, including 2 imported cases, were identified in 1997, and a small-scale outbreak with 9 confirmed cases occurred in the southern part of the island in 1998 (3-6). Seroepidemiological studies conducted in Taiwan, China, in 1993-97 showed that measles IgG seroprevalences were 85-92% in preschool and kindergarten children (2-5 years old) and reached 92-98% in schoolchildren (6-15 years old) and young adults over 16 years old (6-9).

In this study we investigate the probable long-term measles antibody profiles and evaluate the need for policy refinement with or without the assumption of waning vaccine-induced immunity. Using vaccine coverage rates, empirical seroconversion rates and assumed antibody titre decay rates, epidemiological models are developed by which to compare the merits of different vaccination strategies on the island. This will assist in formulating measles elimination strategies there and in other parts of the world where a similar stage in measles elimination has been reached.

Materials and methods

Predicting measles IgG seroprevalence

It is assumed that the measles IgG seroprevalence in preschool Children will be only from vaccination. Initially, it is also assumed that measles vaccine-induced immunity will not wane once seroconversion after vaccination occurs. The current measles vaccination policy is one dose of measles vaccine at 9 months of age and one dose of MMR vaccine at 15 months of age, with an additional an `mop-up' strategy. Kindergarten education is voluntary on the island and the proportion of children attending is not known. Primary education starting from 6 years of age is mandatory and the proportion in attendance is over 99%. Therefore, the possible effect of MMR `mop-up' among newcomers to primary school but not newcomers to kindergarten will be considered. Health statistics on vaccine coverage are classified broadly as coverage for 1 dose of measles vaccine and coverage for 1 dose of MMR. Inclusive data are not available on the proportion receiving 1 dose of measles vaccine and no MMR, or 1 dose of MMR and no measles vaccine, or the proportion receiving 2 doses of vaccination (measles and MMR). …

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