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Dengue vaccine protects people nine years or older — but harms younger kids

The vaccine increased hospitalizations in kids under the age of nine

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A vaccine for dengue, a leading cause of illness and death among children in some Latin American and Asian countries, protects people nine years old and older from the infection, according to a study in the New England Journal of Medicine. There’s a catch, though: children vaccinated before age nine were actually more likely to contract the illness.

About half the world’s population lives at risk of dengue. As a result, about 500,000 people — many of whom are children — are hospitalized every year with the most severe form; about 12,500 die. And what's worse is that the incidence of dengue has grown substantially around the world in the last 50 years. There’s no cure or preventive medicine for the mosquito-borne disease either — which is why this vaccine is so important.

The vaccine protected two out of every three volunteers aged nine or older

Children who were nine or older at the study’s start were 80 percent less likely to be hospitalized from dengue two years later, compared to their peers receiving a placebo. In that group, two-thirds of the kids didn’t contract dengue of any kind. Kids who were vaccinated earlier, though, were more likely to be hospitalized, especially if they were between two and five years old at the study’s start.

It’s not clear why the results were different among the age groups, writes Cameron Simmons, a dengue virus researcher at the University of Oxford who didn't participate in the study, in an editorial accompanying it. "It’s possible the results are chance findings," Simmons writes. But it’s also possible that some children may not have been hospitalized despite being ill. Because the study examined hospitalizations, there may have been illnesses that were missed.

But younger children were more likely to be hospitalized

The study enrolled more than 35,000 youths aged two to 16 in three clinical trials that took place in six Asian and five Latin American countries, including Thailand, Mexico, and Colombia. Most of them received a dengue vaccine developed from Sanofi, a pharmaceutical company; the rest got a placebo. Three to four years later, the researchers checked on the participants.

"Based on the data available today... we conclude that our candidate vaccine will have the greatest public health impact if used in people nine years of age and older," says Melanie Saville, chief medical and clinical officer of dengue research at Sanofi. The vaccine "promises to be a real tool for endemic countries."

It's still too early to determine the cost of the vaccine, Saville says. Right now, Sanofi's focus is getting the drug approved in countries where dengue is prevalent. "The regulatory approval process has started in several countries of Asia and Latin America," Saville says. Once the dengue vaccine is authorized, the company will start to look at how best to administer it.