杏吧原创

Measles strikes where migrating people crowd together. The good news is that you can tell exactly where that is 鈥� and where to aim your vaccination 鈥� by looking from space for the glow of the migrants鈥� lights.

Measles kills around 164,000 unvaccinated children per year. Between 2000 and 2008 an aggressive vaccination campaign in poor countries cut deaths by 80聽per cent globally, and by a massive 92聽per cent in Africa.

Yet in 2008 economic cutbacks hit vaccination, and measles 鈥� the most contagious human virus known 鈥� began to bounce back, with a resurgence in west Africa seen as a particular threat.

When measles was common in Europe and North America, it was known that seasons of the disease followed school calendars, which crowded children together. People in impoverished west Africa also crowd together during winter dry seasons, but this doesn鈥檛 necessarily mean crowding drives the measles that occurs during this season, says at Imperial College London: the virus also survives better in drier weather.

at Princeton University and colleagues have now teased the variables of crowding and weather apart by scrutinising satellite images for the glow of people鈥檚 night-time fires and electric lights.

Frugal lighting

Past research has found that in poor regions, where night-time illumination is sparse enough not to saturate a satellite鈥檚 sensor, its brightness corresponds to population density. Without the images, says Bharti, 鈥渋t is very difficult to measure changes in population density鈥�.

Using satellite images taken between 2000 and 2004, Bharti鈥檚 team mapped dry-season increases in night-time glow in three cities in Niger: Niamey, Maradi and Zinder. The brighter periods corresponded closely to the time of measles outbreaks. In a fourth city, Agadez, where there is similar weather but little seasonal migration, there was no increase in either glow or measles.

The satellite observations were detailed enough to map population density in three different districts of Niamey, Niger鈥檚 capital. The city also had district-level counts of measles cases from an outbreak in 2003 and 2004. The light exactly tracked the different courses of measles outbreaks in the three districts.

If the Niamey health authorities had had this information at the time, they could have responded sooner. They did mount a vaccination campaign in response to the outbreaks, but because of delays in reporting it didn鈥檛 reach two of the three districts until the disease there was already declining. In the third, however, the glow 鈥� and measles 鈥� were still rising. Deaths there were lowest, perhaps because vaccine arrived before many children were exposed.

鈥淲e are working towards making this useful for real-world applications,鈥� says Bharti. The satellite images are available within 48 hours.

Who鈥檚 at risk?

, head of the Institute of Immunology in Luxembourg, cautions that outbreaks are affected by more than just population density 鈥� it also matters how many people are susceptible to measles. 鈥淚ncreased light may not always mean an outbreak, if there aren鈥檛 enough susceptibles.鈥�

This insight into what makes measles seasonal is important, though, says Muller. 鈥淚f we understand that better, we can target vaccination better, to make sure less virus gets through the off season,鈥� he says. If we do that well enough, 鈥渨e can eventually eradicate measles鈥�.

Journal reference: , DOI: 10.1126/science.1210554