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The FDA on Thursday granted accelerated approval to the for adults at increased risk of exposure to the mosquito-transmitted virus, with confirmation of clinical benefit required for a full approval.

Risk of infection with chikungunya virus is greatest in tropical/subtropical regions of Asia and Africa, but virus-carrying mosquitoes are spreading and already endemic in parts of the Americas. Over the past 15 years, upwards of 5 million chikungunya cases have been documented worldwide.

"Infection with chikungunya virus can lead to severe disease and prolonged health problems, particularly for older adults and individuals with underlying medical conditions," Peter Marks, MD, PhD, director of the FDA's Center for Biologics Evaluation and Research, said in a statement. "Today's approval addresses an unmet medical need and is an important advancement in the prevention of a potentially debilitating disease with limited treatment options."

Typically emerging within a week of infection, common chikungunya symptoms include fever, severe joint pain, and rash, and may include headache and muscle pain. In as many as half of all cases, persistent disease can cause disabling polyarthritis. Treatment typically consists of fluids and over-the-counter pain medications.

Approval of the live-attenuated vaccine was supported in part by a phase III placebo-controlled study involving more than 4,000 healthy U.S. volunteers. In that study, a single dose of vaccine -- based on a La Reunion strain of east central South African genotype -- produced a strong immune response in nearly all recipients.

At 28 days post-vaccination, 98.9% (95% CI 96.7-99.8) of participants receiving the vaccine had neutralizing antibody levels shown to be protective against chikungunya disease (in non-human primate studies) versus 0% of placebo recipients (P<0.0001). And seroprotection was consistent across age groups: 98.6% for participants under 65 and 100% for those 65 and older.

As a condition of the approval, evidence will be required to confirm the vaccine's ability to .

In trials, vaccine side effects were similar to those caused by infection with the virus, including headache (32%), fatigue (29%), myalgia (24%), arthralgia (17%), fever (14%), nausea (11%), and tenderness at the injection site (11%).

Postmarketing studies required by FDA will monitor the risk for severe infection-like adverse reactions from vaccination. In trials, severe chikungunya-like side effects occurred in 1.6% of the individuals receiving the vaccine, including two hospitalizations, and some recipients had reactions of this sort lasting a month or more. As such, warns of the risk for severe or prolonged chikungunya-like reactions.

Other warnings and precautions note that vertical transmission of the wild chikungunya virus from pregnant women to newborns can cause severe or fatal disease in some cases, though it's unknown whether the weakened vaccine virus can be transmitted or cause these serious events. Trials have shown undetectable levels of the attenuated vaccine virus 14 days after vaccination.

"When considering administration to pregnant individuals, healthcare providers should take into consideration the individual's risk of exposure to chikungunya virus, gestational age, and risks to the fetus or neonate from disease caused by chikungunya virus in the pregnant individual," the FDA stated.

The vaccine is contraindicated in immunocompromised individuals.

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