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November 1, 2016
Using satellite imagery to improve vaccination coverage
At a Glance
- Researchers analyzed satellite images of nighttime lights to help estimate coverage for a measles vaccination campaign in Africa.
- The approach can help with disease surveillance and planning immunization campaigns that account for seasonal shifts in populations.
Millions of people worldwide die each year from infectious diseases such as measles. This disease is very contagious and can easily spread when an infected person coughs or sneezes. Measles can be prevented with a vaccine, and has almost been completely eliminated in the U.S. due to widespread use of the vaccine.
Measles is still common in many developing countries, including parts of Africa and Asia, where people haven’t been vaccinated. Globally, more than 114,000 people die from measles each year. Effective vaccination campaigns depend on accurate estimates of population size and dynamics. In areas that have seasonal migrations and limitations in data collection, however, such estimates can be difficult.
A research team led by Dr. Nita Bharti from Penn State University used a novel method to assess the coverage of a community immunization campaign during a measles outbreak in Africa. The team focused on an outbreak in Niamey, a city located in the country of Niger. The outbreak resulted in more than 10,000 cases and almost 400 deaths in 2003-2004. The study was supported in part by NIH’s Fogarty International Center (FIC). Results were published online on October 5, 2016, in Scientific Reports.
About 6 months after the start of the outbreak (in April 2004), officials conducted a vaccination campaign that aimed to immunize 50% of children less than 5 years of age. The campaign ended after achieving an estimated 57% coverage in less than 2 weeks. Follow-up surveys, however, revealed that the vaccination coverage was only about 50% due to an underestimate of the target population’s size.
To better understand and quantify the fluctuations in population size and distribution during this time, the researchers analyzed visible and thermal-infrared images obtained from orbiting satellites. The images had a spatial resolution of about 1 kilometer, allowing analysis of brightness within cities. The team screened images that were taken between 7-10 pm and that weren’t affected by bright moon phases or clouds. These images thus captured nighttime lights from electric lighting and fires.
Modeling of the satellite images showed that the city’s total population size was larger than estimated. The team determined that 11,000 more children in the target vaccination age had been present in the city at the time of the outbreak than had been estimated. The immunization campaign in Niamey had occurred during the dry season, when the population had increased with seasonal migrants.
“Human movement and gathering have been a big part of understanding the epidemiology of directly transmissible diseases like measles,” says Dr. Matthew Ferrari, a member of the research team. “What was exciting about this project was to turn that phenomenon into a potential solution. Rather than looking at times of large gatherings—harvest season, or cultural festivals—as high risk periods, we can look at them as opportunities to serve people who are normally beyond the reach of conventional health systems.”
—by Carol Torgan, Ph.D.
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References: Measuring populations to improve vaccination coverage. Bharti N, Djibo A, Tatem AJ, Grenfell BT, Ferrari MJ. Sci Rep. 2016 Oct 5;5:34541. doi: 10.1038/srep34541. PMID: 27703191.
Funding: NIH’s Fogarty International Center (FIC) and National Institute of Allergy and Infectious Diseases (NIAID); Branco Weiss–The Society in Science; Bill and Melinda Gates Foundation; Huck Institute of Life Science at Penn State University; Department of Homeland Security; and Wellcome Trust.