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Measles Incidence Before and After Supplementary Vaccination Activities --- Lusaka, Zambia, 1996--2000
Zambia is a sub-Saharan African country (2000 population: nine million) with approximately 10% of the population residing in the capital of Lusaka. In Zambia, measles is one of the five major causes of morbidity and mortality among children aged <5 years. During 1991--1999, the annual number of reported measles cases ranged from 1698 to 23,518. In August 1999, supplementary vaccination activities (SVAs) were conducted in Lusaka among children aged 9 months--4 years. This report summarizes measles incidence, measured by the number of patients presenting to selected medical facilities, before and after SVAs and suggests that substantial measles transmission continued despite this intervention. To improve measles control in Zambia, nationwide supplementary measles vaccination is planned for children aged 9 months--14 years in 2002.
The routine vaccination program in Zambia includes one dose of measles vaccine administered at age 9 months. Reported national measles vaccination coverage ranged from 93% in 1996 to 72% in 1999, with wide fluctuations among districts. In Lusaka, reported vaccination coverage decreased from >95% in 1996 to 54% in 1999 (Ministry of Health, Zambia, unpublished data, 1999).
To accelerate measles control, SVAs were conducted in four urban districts (Kabwe, Kitwe, Lusaka, and Ndola) that comprised approximately one fourth of the Zambian population. During August 20--23, 1999, measles vaccine for children aged 9--59 months, vitamin A for children aged 6--59 months, and oral poliovirus vaccine for children aged 0--59 months were administered during the second round of polio subnational immunization days. Measles vaccine was administered to 197,077 children regardless of prior measles vaccination or disease history. The reported measles vaccination coverage for the four urban districts combined was 81%; Lusaka district reported coverage of 83% (1).
To assess the results of the 1999 campaign, a field investigation was conducted in Lusaka district. Attendance registers were reviewed for patients with measles seen during August 1996--September 2000 at the main city hospital and three health-care centers located in different areas of the city. Data on age, date of disease onset, date of admission, and mortality were abstracted. Because measles in partially immunized populations is a seasonal disease characterized by periodic epidemics, the impact of SVAs was assessed by comparing the annual number of measles cases, deaths, and the age distribution of these before and after SVAs. Three consecutive 12-month periods before SVAs were compared with one 11-month period after SVAs. The post- SVA period started 1 month after the vaccination campaign was conducted (i.e., September 23, 1999--August 22, 2000).
From September 23, 1996, through September 22, 1999, 2048 measles cases were recorded in Lusaka. The highest monthly incidence occurred during October 1996 and October 1998 (Figure 1). Case counts for the pre-SVA periods during 1997, 1998, and 1999 were 900, 333, and 815, respectively; 496 cases were recorded during the post-SVA period.
Of the 2048 patients with measles during the pre-SVA period, 869 (42%) were aged 1--4 years (Table 1). Following SVAs, among the 496 measles patients, 144 (29%) were aged 1--4 years (Chi-square test, p<0.001). The number of measles cases among persons aged >15 years increased in each successive study period (Table 1). The age distribution of measles patients was similar for both inpatients and outpatients. For the four study periods, clinical outcome (e.g., death) was available for 239 (27%) of 900 (1997), 249 (75%) of 333 (1998), 539 (66%) of 815 (1999), and 294 (59%) of 496 (2000) patients, respectively. Among patients with known outcome, 15 (6%), 22 (9%), 42 (8%), and 18 (6%) died during the four study periods. From September 23, 1996, through September 22, 1998, no measles deaths were recorded among persons aged >10 years; two deaths and three deaths were recorded in this age group in the two latter study periods, respectively (Table 1).
Reported by: A Mtonga, E Synyinza, J Nyrenda, Central Board of Health; M Banda, Lusaka District Management Team, Lusaka, Zambia. World Health Organization Office for Eastern Africa, Nairobi, Kenya. Vaccine Preventable Disease Eradication Div, National Immunization Program; and an EIS Officer, CDC.
During 1989--1990, the World Health Assembly and the World Summit for Children set goals of reducing measles morbidity by 90% and mortality by 95% compared with prevaccine estimates (2,3). Despite these goals and the existence of safe and effective measles vaccines for approximately 35 years, an estimated 30 million cases and 875,000 deaths are attributed to measles each year (4). In March 2001, the World Health Organization (WHO)/United Nations Children's Fund Global Strategic Plan established a goal of reducing global measles deaths by 50% by 2005 compared with 1999 levels (5). Strategies to decrease measles deaths include 1) achieving and sustaining high population immunity through vaccination; 2) enhancing measles surveillance with integration of epidemiologic and laboratory surveillance; and 3) improving measles case management. The plan recommends that a second opportunity for measles vaccination be offered to all children either through regular SVAs or as a second dose in the routine vaccination schedule if coverage with the first dose of measles vaccine is >90%.
Although SVAs in Lusaka did not have a major impact on measles morbidity and mortality during the 11-month period following the intervention, the expected seasonal peak during September--December 1999 appears to have been blunted and the proportion of cases among persons aged 1--4 years was reduced. SVAs had limited impact for two major reasons. First, vaccination coverage during SVAs was <85%, and reported coverage may have overestimated actual coverage. In Burkina Faso, cluster surveys in six urban districts after SVAs in 1998 indicated that measles vaccination coverage was 15%--52% lower than reported coverage (6). Second, routine coverage declined during 1997--1999. Conducting SVAs in a setting where routine coverage is declining results in an increase in the number of susceptible infants.
Other possible reasons for the limited impact of SVAs in Lusaka are 1) only children aged 9--59 months were targeted for vaccination, and approximately 20% of reported cases occurred among persons aged >5 years; and 2) SVAs were limited to urban areas. Preliminary data suggest that, because of the high contagiousness of measles and migration of susceptible persons from rural areas, targeted urban campaigns have limited impact on transmission, especially during epidemics (World Health Organization Office for Eastern Africa, unpublished data, 1999).
At least four factors contributed to low coverage during SVAs in Lusaka. First, measles vaccine and injection equipment arrived late (1 day before the start of the second round of polio subnational immunization days). Second, donor funds for operational costs were delayed, resulting in insufficient funds for personnel and fewer vaccination posts. Third, health-care workers went on strike on one of the campaign days because of nonpayment of the full government allowances. Finally, supervision and monitoring were inadequate at the central and district levels (1).
During the 11-month period following SVAs, six measles deaths (33% of the annual total) occurred among children who should have received measles vaccination during the campaign. The increase in the number of measles cases among older persons in the latter two study periods may be the result of migration of susceptible persons into Lusaka or changes in use of health-care facilities included in the study.
Improvements in the vaccination infrastructure in Zambia, a reversal of the declining trend in routine vaccination coverage, improvements in monitoring of coverage, high coverage (>95%) in future SVAs that target a wider age group and geographic area, and strengthening of surveillance are needed to decrease measles-associated morbidity and mortality in Zambia. Advocacy and improved partner coordination are needed to further reduce measles morbidity and mortality.
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