Dengue fever has surged in Brazil’s hot rainy season, forcing health authorities to take emergency measures and start mass vaccination against the mosquito-borne illness.

In the first five weeks of this year, 364,855 cases of infection have been reported, the health ministry said, four times more than dengue cases in the same period of 2023.

The rapid spread of dengue has caused 40 confirmed deaths, the ministry said, and a further 265 are being investigated.

Brazil has bought 5.2m doses of the dengue vaccine Qdenga, developed by Japanese drugmaker Takeda, with another 1.32m doses provided at no cost to the government, a ministry statement said.

Three Brazilian states have declared emergencies, including the second most populous state, Minas Gerais, and the Federal District, where the capital, Brasília, is located and is facing an unprecedented rise in infections.

Brasília will start vaccinating children aged 10-14 on Friday with Qdenga, the local government said on Wednesday.

Cases of dengue in Brasília since the start of the year have exceeded the total for the whole of 2023, with a rate of infection of 1,625 cases per 100,000 inhabitants, compared with the national average of just 170.

Army troops have been deployed in the capital to help track breeding spots of the Aedes aegypti mosquito that carries and spreads the dengue virus in homes and backyards wherever there is stagnant water.

On Monday, the Brazilian air force set up a field hospital in preparation for a surge in cases needing hospital care in Ceilândia, a densely populated poor suburb of Brasília.

Cities such as Rio de Janeiro that are preparing to celebrate Carnival starting on Saturday have taken measures to prevent an epidemic.

The health ministry has set up an emergency center to coordinate operations against dengue across Brazil.

The World Health Organization (WHO) director-general, Tedros Adhanom, visiting Brasília on Wednesday, said this dengue outbreak has been fuelled by the El Niño phenomenon that brought increased rainfall in Brazil.

“This current dengue outbreak is part of a large global increase in dengue fever with over 500m cases and over 5,000 deaths reported last year from 80 countries in every region of the world except Europe,” he said at a ministry event.

South America is seeing a surge in cases of dengue during the southern hemisphere summer, exacerbated by rising temperatures and the El Niño weather pattern in the Pacific that contribute to prolonged dengue seasons and spread of infections, scientists say.

Dengue fever symptoms include a high fever, headache, vomiting, muscle and joint pains, and an itchy skin rash. In some cases, the disease can cause a more severe haemorrhagic fever, resulting in bleeding that can lead to death.