IJE Advance Access originally published online on September 19, 2005
International Journal of Epidemiology 2005 34(6):1443-1444; doi:10.1093/ije/dyi197
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Letters to the Editor |
Migration and measles
1 Division of Cardiology, Department of Medicine, University of Cape Town, E25 New Groote Schuur Hospital, Observatory 7925, South Africa
2 Expanded Programme on Immunisation, Ministry of Health, Cameroon
3 World Health Organization, Country Office, Yaoundé, Cameroon
4 Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, South Africa
* Corresponding author. E-mail: wiysonge{at}yahoo.com
Sirs—The study by Yaméogo et al.1 highlights the importance of understanding population movement when determining the optimal timing and geographical extent of a measles supplementary immunization activity (SIA). A wide age-range SIA usually reaches children who could not be reached by systematic immunization services, rapidly reducing the proportion of measles susceptible individuals in the population, resulting in a significant reduction in measles incidence and lengthening of the time interval between major measles outbreaks.2 Yaméogo et al.'s study shows that the time to a major outbreak following an SIA is not only determined by the vaccination coverage achieved, population density, and birth rate3,4 but also population movement. Our experience with measles control in Cameroon is consistent with this finding that, in our opinion, has important policy implications.
Cameroon is described as ‘Africa in miniature’ because of its diverse natural environment, stretching from the dense equatorial forest zone in the south through the mountainous grass fields to the Sahel zone of the north. In 2002, close to 7.3 million (92%) children aged nine months to under-fifteen years of age were vaccinated during measles SIA in the country. Following the SIA, basic demographic and clinical information are obtained for each patient presenting with rash and fever and one venous blood specimen drawn for serological confirmation of measles infection in the laboratory, according to World Health Organization guidelines.5 Prior to the SIA, from 1995 to 2001, systematic measles vaccination coverage was <70% in all provinces. During this period, as shown in Figure 1, major measles outbreaks occurred every year in the three provinces of the Sahelian north (with high population movement to and from neighbouring Nigeria and Chad) and every 2–3 years in the seven provinces of the south (with less inter-country population movement).
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The three northern provinces contributed 53.9% (45 694/84 703) of the total measles cases in Cameroon during the seven immediate pre-SIA years. Following the measles SIA, which has reduced measles cases in Cameroon by 98.5% from 23 934 in 2001 to 358 in 2004, nearly all the measles cases in the country are now reported by these northern provinces (Figures 2 and 3). In 2003 and 2004, these provinces contributed 90.1% (209/232) and 93.3% (334/358) of the annual measles cases in Cameroon, respectively. The Far North Province reported 154 cases in 2003, with the frontier districts of Kolofata and Kousseri in the province reporting 82 (26.4%) and 31 (10.0%) of the cases, respectively. In 2004, the province reported 311 measles cases, 197 (63.3%) from Kolofata and 73 (23.5%) from Kousseri. The proportion of measles cases that either indicated residing permanently in a neighbouring non-Cameroon village or Cameroonians who were (in religious schools) across the border in the months preceding consultation for measles was 33.6% (78/232) in 2003. None of the 78 cases was vaccinated while 12.3% (19/154) of the cases resident in Cameroon had received a measles vaccine at least 1 month before the onset of symptoms. The Far North Province reported 74 of the cases, Kolofata reported 70 cases. In 2004, 46.4% (166/358) of measles cases were reportedly resident outside Cameroon before the onset of measles, all of them from the Far North Province and 160 from Kolofata. Of the cases who reported residing outside Cameroon <1% (1/166) were vaccinated compared with 7.3% (14/178) of the ones who were resident in Cameroon.
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These observations show that cross-border migration plays a key role in measles epidemiology in Cameroon and, by implication, that the current significant strides in accelerated measles control will not interrupt measles transmission in the country without simultaneous improvement in measles control in neighbouring countries. We, therefore, heartily welcome the imminent synchronization of multi-country measles SIA in West Africa;1 for measles, like the polio,6 virus knows no political boundaries.
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References |
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 Top References |
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1 Yameogo KR, Perry RT, Yameogo A et al. Migration as a risk factor for measles after a mass vaccination campaign, Burkina Faso, 2002. Int J Epidemiol 2005;34:556–64.
2 de Quadros CA, Izurieta H, Venczel L, Carrasco P. Measles eradication in the Americas: progress to date. J Infect Dis 2004; 189(Suppl. 1):S227–35.
3 Anderson RM, May RM. Immunisation and herd immunity. Lancet 1990;335:641–45.[CrossRef][ISI][Medline]
4 Finkenstadt B, Keeling M, Grenfell B. Patterns of density dependence in measles dynamics. Proc Biol Sci 1998;265:753–62.
5 World Health Organization Regional Office for Africa. Guidelines for measles surveillance. Harare: AFRO, 2003.
6 Global Polio Eradication Initiative. Polio Eradication Situation Report—April 2005. Available at: http://www.polioeradication.org/ (Accessed April 20, 2005).
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