Bad weather in sub-Saharan Africa increases the spread of HIV, according to a study published in the June 2015 issue of the Economic Journal. When the rains fail, farmers in rural areas often see their incomes fall dramatically and will try to make up for it however they can, including through sex work. Analysing data on more than 200,000 individuals across 19 African countries, the research team finds that by changing sexual behaviour, a year of very low rainfall can increase local infection rates by more than 10%.

The results have important policy implications for fighting the spread of the epidemic, as co-author Erick Gong of Middlebury College notes:

''Existing approaches to stopping the spread of HIV – such as promoting condom use and the use of anti-retrovirals – remain critically important.

''But our results suggest that other policy approaches could be very useful too – in particular, approaches that provide safety nets to rural households when the weather turns bad.''

Policies and investments seemingly unrelated to HIV – such as the promotion of rural insurance or household savings schemes, or the development of drought-tolerant crops – might have surprising benefits in slowing the HIV epidemic. Co-author Kelly Jones of the International Food Policy Research Institute says:

''The HIV/AIDS epidemic remains one of the world''s greatest health challenges, with over a million new infections per year in Africa alone.

''Our results expand the menu of options for addressing the epidemic, and highlight some surprising options that are not at the forefront of people''s minds.''

The research sheds valuable light on why HIV continues to spread in Africa. Previous studies have documented in limited settings that poor women often alter their sexual behaviour in response to an income shortfall. But until now, there has been little evidence that this response is big enough to affect the trajectory of the HIV epidemic.

To fill this gap, the researchers combined data on the HIV status of thousands of people across sub-Saharan Africa with data on the recent rainfall history in each individual''s location.

Because years of low rainfall can lead to much lower incomes in these locations, particularly in rural areas where people depend more heavily on agriculture for their livelihoods, variation in rainfall provides a way to study how changes in local economic conditions affect infection rates. Co-author Marshall Burke of Stanford University comments:

''We were surprised by how strong the relationship is between recent rainfall fluctuations and local infection rates.

''As expected, the relationship is much stronger in rural areas, and particularly for women who report working in agriculture. These are the people who really suffer when the rains fail, and who are forced to turn to more desperate measures to make ends meet.''

''Income Shocks and HIV in Africa'' by Marshall Burke, Erick Gong and Kelly Jones is published in the June 2015 issue of the Economic Journal. Marshall Burke is an assistant professor of environmental earth system science at Stanford University. Erick Gong is an assistant professor of economics at Middlebury College. Kelly Jones is a research fellow at the International Food Policy Research Institute (IFPRI).