The Food and Drug Administration has approved the first vaccine for dengue, Dengvaxia, but placed significant restrictions on its use because the vaccine has been shown to put some people at heightened risk for a severe form of the disease.

In clearing the vaccine, the agency acknowledged the serious public health benefit of slowing a disease that affects hundreds of millions of people around the world. The decision may also help a struggling product whose use has stalled because of concerns over its possible risks.

“It tells the world that if properly used, the vaccine can be effective,” said Duane Gubler, an emeritus professor at Duke-NUS Medical School who is one of the inventors of a competing dengue vaccine made by Takeda, and has consulted in the past for Sanofi, which makes Dengvaxia.

Sanofi has sold Dengvaxia overseas since 2015, but the vaccine hit a major roadblock in 2017 after the Philippines, which had widely distributed the product to schoolchildren, halted its use and revoked Sanofi’s license.

That decision came after Sanofi announced that in rare cases, if people who never had dengue were vaccinated and later became infected, the vaccine might provoke a much more severe form of the illness.

The FDA earlier this month limited its approval to people ages 9 to 16 who live in areas where dengue is endemic and who are shown by lab testing to have already been infected with the disease.

More than one-third of the world’s population lives in areas at risk for infection with the dengue virus, according to the Centers for Disease Control and Prevention. The virus is spread by mosquitoes.

The illness, also called breakbone fever, can be excruciating, with high fevers, headaches, muscle and joint pains, and lingering weakness. A second infection of dengue can lead to a severe form of the disease, which can cause hemorrhage or shock and can be fatal.

An estimated 400 million dengue virus infections occur around the world, and there are about 500,000 cases of the severe form, dengue hemorrhagic fever, which causes about 20,000 deaths, according to the CDC. No drugs are approved to treat dengue disease.

According to the CDC, most cases of dengue fever in the 48 contiguous states were acquired elsewhere by travelers or by immigrants, although some isolated outbreaks have occurred, such as in South Texas in 2005. Dengue is endemic in the U.S. territories of American Samoa, Guam, Puerto Rico and the Virgin Islands.

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