CDC Activates Emergency Response as Florida Faces Largest Dengue Outbreak in Decades

The Centers for Disease Control and Prevention (CDC) has launched a Level 3 emergency response to address dengue outbreaks in Florida and the Republic of the Marshall Islands, marking the agency’s eighth emergency activation this year.

Florida is experiencing its most significant dengue outbreak in decades, with 745 cases reported in 2024. Of those, 425 were locally acquired—the highest number in the continental United States since 1935—with the majority concentrated in the Tampa Bay area.

“Given the widespread presence of Aedes aegypti mosquitoes and recent hurricane-related rainfall, continued domestic transmission remains a concern,” wrote Chris Braden, MD, acting director of the CDC’s National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), in an email shared with MedPage Today.

Braden acknowledged the extraordinary workload facing NCEZID staff, who have managed responses to Cyclospora, Ebola, measles, hantavirus, and New World screwworm outbreaks this year, along with the World Cup safety activation and the Potomac River wastewater spill response near Washington, D.C.

The Marshall Islands has declared a state of emergency on its two largest islands, Ebeye and Majuro. The CDC has provided programmatic support and deployed an Epi-Aid team to assist.

During a Thursday clinician call, CDC experts reported that locally acquired cases in Florida primarily involve the DENV-2 serotype. Approximately 54% of cases occur in men, with a median age of 43; 45% of infections affect adults aged 30 to 49. Nearly half of all cases—47%—required hospitalization, a figure that may signal substantial underreporting.

While locally acquired dengue is not unprecedented in Florida, the scale of this year’s outbreak is notable. Historically, the continental U.S. lacked the mosquito species capable of sustaining transmission, explained Adam Waickman, PhD, a dengue expert at SUNY Upstate Medical University. The expanded range of Aedes aegypti—now found as far north as Virginia—and Aedes albopictus, which reaches Pennsylvania, has changed that dynamic.

Waickman emphasized that 70% to 90% of dengue infections are asymptomatic, meaning reported cases represent only a fraction of actual infections. Traditional mosquito spraying programs are largely ineffective against these species, which thrive in and around homes in urban environments.

“Wearing mosquito repellent, covering up with long sleeves—those are the most effective personal protections,” he advised.

No dengue vaccines are currently licensed in the United States. Sanofi Pasteur’s vaccine was found to increase the risk of severe disease in dengue-naïve individuals through antibody-dependent enhancement. Takeda’s vaccine, approved in the European Union, the U.K., and parts of Latin America and Asia, withdrew its U.S. biologics license application in 2023 following FDA discussions on data collection—months before dengue cases surged in Puerto Rico.

Several antivirals and monoclonal antibodies are in clinical trials, with one monoclonal antibody showing promise in a Phase II trial for hospitalized patients. Waickman stressed that a combination of vector control, vaccines, and therapeutics will be essential as dengue continues to spread and likely becomes endemic in the U.S.

“The cat is out of the bag,” he said. “It’s only going to keep spreading.”

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