When female Aedes Aegypti mosquito sups on the blood of its human victims it too often deposits the virus that causes dengue, causing as many as 400 million infections per year worldwide. Severe forms of the painful, flu-like disease can be fatal, especially among children. And until recently there has been no truly effective prevention except avoiding getting bit.

But the outlook against the disease is looking better.

During the past month Dengvaxia, developed by the French pharmaceutical company Sanofi, has been approved for use in three countries: Mexico and the Philippines approved the vaccine earlier this month. This week, the company also announced the drug has received the green light in Brazil, which has seen more than 1.4 million cases of the disease in 2015. Exactly when the inoculations will be deployed—and at what price—remains unclear as terms of the vaccine are being negotiated between the company and the countries.

Sanofi’s vaccine, which is designed to coax the body’s immune system into making antibodies against all four forms of dengue, is a live virus comprised of an attenuated yellow fever virus. (Yellow fever virus and dengue virus have the same genus.) For the vaccine, however, the virus is genetically engineered to include genes encoding for dengue proteins. Other dengue vaccines are also in development but none have received approval.

The need for an effective vaccine is evident. The mosquito-borne disease has a massive and growing reach. Dengue has become an increasing threat around the world as both the range of the disease-carrying mosquitoes has grown and people travel to dengue-endemic locations. Health officials have also become better at diagnosing the disease. Dengue has been reported in Japan after a lapse of about 70 years. The number of cases has grown in Brazil and its neighbors. Meanwhile, in the United States, there was an outbreak this year in Hawaii and the disease is already endemic in Puerto Rico.

That’s not to say Dengvaxia is a perfect vaccine. In clinical trials it only reduced the chances of developing the disease by about 60 percent. Also, it is only approved for use in people nine to 45 years old who live in dengue-endemic areas —not young children or the elderly. In fact, the vaccine seems to be least effective in children younger than nine years old, particularly among kids under 6, whose immune systems are especially vulnerable and might be part of the group who need the vaccines most. There are also unanswered questions regarding vaccinated individuals who could potentially have more severe cases of the disease if they contract it later in life.

Even as Brazil and other tropical countries attempt to tamp down dengue with mosquito control and public health campaigns, they are also facing other mosquito-borne threats including chikungunya and, increasingly, zika. The latter disease, which also causes dengue-like symptoms, has recently been linked with babies of infected pregnant mothers being born with abnormally small heads, or microcephaly, in Brazil. Researchers have detected an alarming, inexplicable uptick in such cases in the northeast part of the country. That’s in addition to earlier reports, documented by Scientific American, linking zika to another condition that can cause paralysis.

From the U.S. perspective it remains unclear how a vaccine would be used domestically, whether it would be used in areas that already have seen dengue including Hawaii or Florida or perhaps among those traveling to dengue-endemic countries. By law, the U.S. Food and Drug Administration cannot comment about if they are reviewing an application for the vaccine to be used in the U.S. But Harold Margolis, chief of the dengue branch at the U.S. Centers for Disease Control and Prevention, says the vaccine does appear to be safe and well-tolerated in clinical trials. Still, he has some reservations. “The vaccine has significantly lower efficacy with dengue 1 and 2 than dengue 3 and 4,” he says, referring to the distinct but closely related strains of the virus. The vaccine may also perform differently in larger settings and communities than it did in clinical trials, he says. In April 2016, World Health Organization immunization advisers will examine the vaccine and provide recommendations for its use. Ultimately, Margolis says, "the U.S. is interested in having a vaccine that prevents disease."