POPULATION IMMUNITY TO MEASLES IN ZAMBIAN CHILDREN

Sunday, 22nd of February 2009

CSU 9/2009: Population Immunity to Measles in Zambian Children

 Writing in a recent number of  The Lancet, Sara Lowther and colleagues show absence of measles vaccination antibodies in a significant minority of Zambian children three years after a nationwide measles campaign. Here is the authors' discussion:

 

[three years after the campaign] only 84% of children within the study townships had a history of measles vaccination and 67% had detectable antibodies to measles virus in oral fluid samples, suggesting a build-up of susceptible children and a population at risk for measles outbreaks. After adjustment for the reported sensitivity and specificity of the oral fluid assay, the estimated proportion of children with detectable antibodies to measles virus was 70%. In Zambia, the number of reported measles cases increased from 35 in 2004 (the year after the SIA) to 459 in 2006, and a follow-up SIA was done in July, 2007. One of the challenges to continued progress in reducing measles incidence and mortality is the need for repeated mass measles vaccination campaigns. These campaigns require the sustained commitment of resources and personnel, are typically done within 3—4 years of the initial campaign, and target a narrower age-group than initial catch-up campaigns. Cross-sectional surveys such as this, using oral fluid specimens and satellite images for sampling, could be useful to identify the optimum timing of repeat SIA before large outbreaks of measles occur.

 

Poor initial seroconversion would have been a convincing explanation in a population all vaccinated at 9 months of age. But mass follow-up campaigns have typically covered children aged up 59 months of age. So there must be something else at work here, both in the HIV seronegative and seropositives populations tested.

 

On the methodological side, the editors note that '[t]his useful study, which scores high on the novelty front, shows that cross-sectional surveys, using oral fluid specimens and satellite images for sampling, may be useful in determining the optimal timing of repeat supplementary immunisation activities before large outbreaks of measles occur.'

 

It is remarkable that, with case based surveillance including lab back-up, Zambia still registered an annualized measles incidence of less than 1 per 100,000 in the period January-October 2008 (WHO/AFRO presentation, Task Force on Immunization, December 2008). In a country without vaccination, the incidence rate would roughly match the birth rate, or 3000 to 4000/100,000.

 

As Africa region moves towards pre-eradication, studies of this kind, complemented by case based surveillance, will assist governments and partners in reviewing and, as necessary, refining current international recommendations for the timing of follow-up campaigns.