A new study shows a vaccine designed to prevent dengue fever could actually be making people sick.

Dengue infects an estimated 400 million people worldwide each year. The virus spreads to human beings through the bite of an insect, the mosquito.

Most people who get dengue suffer health problems like those resulting from a mild case of influenza, like a higher than normal body temperature.

But the illness is more severe the second time a person gets it. In serious cases, it can cause bleeding inside the body and even death. About 25,000 people a year die from the infection.

The drug Dengvaxia is so far the only vaccine approved for use against dengue. Several countries - including Brazil, Mexico, Costa Rica, Paraguay and the Philippines - have already approved use of the drug. But health officials are still trying to decide the best way to release the vaccine.

Trials of Dengvaxia involved more than 30,000 people in 10 countries. Among young people, the drug was found to reduce infections by 60 percent, and hospitalizations by 80 percent. But over time, many of those vaccinated got seriously ill with dengue.

Researchers from Britain and the United States studied the results of the tests. The researchers were from Johns Hopkins Bloomberg School of Public Health, Imperial College London and the University of Florida. They found that the vaccine acted as a first infection in people who had not had the virus before.

The researchers reported their findings in the journal Science.

The researchers say they are concerned that people who get infected a second time after the vaccine can get severely sick. This could explain why young children especially were getting ill. They had not lived long enough to get a first infection.

Based on the findings, the World Health Organization is warning health care workers not to give the vaccine to anyone younger than nine.

But data showed that people other than children were getting severe dengue cases if they had the vaccine, but were not infected before they received it.

"What we suggest is ... maybe having been exposed to dengue in the past is more important than age itself," said Isabel Rodriguez-Barraquer of the Bloomberg school.

The study found that the vaccine should not be a problem in countries with many cases of dengue. In such areas, Dengvaxia was found to reduce severe sickness and hospitalizations by 20 percent to 30 percent.

But the researchers warned that in countries where only 10 percent of people have been infected with dengue, the vaccine needs to be carefully used. Ideally, Rodriguez-Barraquer said, people should be tested before getting vaccinated, to see if they have been infected in the past.

The researchers are also looking forward to trial results from additional new dengue vaccines.

I'm Bryan Lynn.

Jessica Berman reported this story for VOA News. Bryan Lynn adapted it for Learning English. George Grow was the editor.