A week after the US Food and Drug Administration approved the first vaccine for dengue, Dengvaxia, Tanzania is working to contain the spread of an outbreak that was first reported last month, with increased surveillance.

According to the Ministry of Health about 1,222 people have been diagnosed with dengue fever in Dar es Salaam, Dodoma and Singida Regions, a steep rise from the 304 cases reported in April. Two people have since died.

Deputy Minister of Health Dr Faustine Ndugulile said Dar es Salaam leads with 1,145 patients, Tanga follows with 75 patients and Singida so far has registered one patient with the fever.

This year is the worst dengue outbreak compared with 2014 when more than 400 patients in Dar es Salaam were diagnosed with the disease.

FDA—the US agency responsible for protecting public health by ensuring the safety, efficacy, and security of human and veterinary drugs, biological products, and medical devices—approved Dengvaxia but placed restrictions on its use because the vaccine has been shown to put some people at heightened risk for a severe form of the disease.

The disease affects hundreds of millions of people around the world, dengue is transmitted by female mosquito Aedes aegypti, the same species that transmits yellow fever and the Zika virus. Dengue is said to affect about 390 million people in the world every year.

The illness, also called breakbone fever, can be excruciating, with high fever, headaches, muscle and joint pains, and lingering weakness.

A second infection of dengue can lead to a severe form of the disease, which can cause haemorrhage or shock and can be fatal.

Kemri researchers blame the spread of dengue on world trade, climate change, urbanisation, mosquito habits and resistance to insecticides.

Eggs of the dengue-causing species can survive long periods under dry conditions thus encouraging their transportation over great distances in ships, even across continents.

— Additional reporting by The New York Times