The first-ever vaccine approved for dengue, if not used properly, could make people seriously ill instead of giving protection, reports a study published in the journal Science on Sept. 2.

About 400 million people in about 120 countries are infected by dengue every year, of which more than 2 million people suffer a potentially fatal hemorrhagic fever. However, the dengue vaccine manufactured by Sanofi Pasteur is currently licensed for use in six countries only.

The mosquito-borne infection is caused by not just one, but four different strains of dengue virus. In the case of dengue, the first infection is usually milder while exposure to the virus for the second time is often severe and potentially fatal. This is why developing an effective vaccine has long been a challenge for researchers - an inadequate vaccine could in turn make people sicker.

For the purpose of the study, scientists from the University of Florida, Johns Hopkins Bloomberg School of Public Health and Imperial College London analyzed data involving 30,000 people who participated in vaccine trials carried out in 10 countries.

It was observed from the study results that the vaccine reduced illness and hospitalization of the patients by 20 to 30 percent in areas exhibiting high incidence of infection. On the contrary, the vaccine increased the rate of illness and hospitalization in areas where the incidence of infection was relatively low.

"If this vaccine is used correctly, many people could be spared illness and hospitalization from dengue," said Isabel Rodriguez-Barraquer, a research associate at the Bloomberg School. "But we should make sure we only use it in places where our data suggest it will do more good than harm."

When the vaccine is administered to a person previously exposed to dengue infection, the body considers the vaccine as a second infection. The vaccine in turn stimulates the immune system without severe symptoms as in a natural second exposure.

On the other hand, when the vaccine is given to people unexposed to dengue, the body considers it as first infection. Eventually, when the individual is exposed to the virus again in natural settings, the infection becomes more severe as the body responds as if it is encountering a second exposure.

Derek Cummings, co-author of the study and biology professor at the University of Florida and the Bloomberg School, said that since the impact of the vaccine is unclear, care should be taken in deciding where and how the vaccine be used.

Photo: Pan American Health Organization | Flickr

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