Dengue fever, a mosquito-borne virus, has killed at least 21 people out of 22,000 infected in Vietnam so far this year, an official said Thursday.

The number of people killed by the virus is 25 per cent lower than the same period last year and the number of people infected is 14 per cent lower, according to Nguyen Huy Nga, director of the Ministry of Health's Preventive Medicine Department.

'We predict that the number of people affected and killed by dengue fever this year will be lower than that of last year, though the peak of the dengue fever season is not until August and September,' Nga said.

Nga that said most of the people infected and killed were from the country's southern provinces and that 60 per cent of them were under 15 years old.

'There is no vaccine for the disease, and the most effective way to prevent the disease is to kill mosquitoes,' Nga said.

The official said the age group prone to dengue fever is getting higher. Ten years ago, 90 per cent of the dengue fever cases were under 15 years old, but now it is only 60 per cent.

In 2007, dengue fever infected more than 100,000 people and killed nearly 90.