Amid criticism, Brazil this month gave the OK to its first commercial release of genetically modified mosquitoes in an effort to combat dengue fever.

The World Health Organization estimates that dengue, whose main vector is the aedes aegypti mosquito, threatens over 40% of the world’s population, with 2.35 million cases of the infection reported in 2013 in the Americas alone.

GM aedes aegypti mosquitoes were developed by British biotechnology company Oxitec, which has had a years-long project underway in Brazil. The male mosquitoes are engineered so that when they reproduce with the biting females in the wild their offspring die before they reach adulthood, theoretically decreasing the population and thereby curbing the sometimes fatal infection.

Some environmental and civil society groups issued a statement warning that not enough is known about the human health and environmental consequences of the GM mosquitoes to warrant their commercial release.

“There are no data showing that this GM mosquito actually reduces dengue incidence,” stated Gabriel Fernandes, advisor with the Brazilian ecological organization AS-PTA.