Florida is experiencing a surge of reported cases of local transmission of dengue, including one fatality, with experts seeing longer-term trends likely to increase the global spread of this viral infection.

As of the 38th week of this year (September 20-26), there were 252 reported cases of locally acquired dengue in Florida, compared with 40 cases and 37 cases at this point, respectively, in 2025 and 2024.

This pattern is the reverse of what has happened in recent years with Florida’s cases of travel-acquired dengue. Given Florida’s close ties to the Caribbean and South America, where dengue is more prevalent, the state has long seen cases of people experiencing infections acquired abroad. 

During the historic record global outbreak in 2024 of more than 14.6 million cases reported worldwide, travel-acquired dengue soared in Florida. Reported cases in the state hit 539 as of the 38th week of 2024, up from 316 by that point in the previous year. The equivalent number for this year is 245 travel-related cases.

With a spike in locally acquired cases this year, state and county officials in Florida are stepping up efforts to combat the mosquitoes that spread the disease.

Emptying standing water around the home is critical to reducing growth of the mosquito population, said Kristi Miley, PhD, a researcher at the University of South Florida College of Public Health Center for Global Health and Infectious Disease Research in Tampa, Florida. She advises people to check places like flowerpots, children’s toys, buckets, gutters, and old tires.

At this time, there is not a dengue vaccine available in the US. There are several reasons for this, including dengue’s historic relatively low prevalence nationwide.

In a nation like Brazil, though, there’s a critical need for a vaccine, Miley said. In the 2024 outbreak, more than 10 million dengue cases were reported, including more than 6000 deaths.

“Brazil is a community that definitely can benefit from a vaccine because they probably have a large proportion of the population that’s already been infected” with at least one kind of dengue, said Miley.

Initial dengue infections tend to be mild, with symptoms like headaches, if people have any symptoms. An estimated 40%-80% of infections are asymptomatic. Many people are unaware they have had a dengue infection. But subsequent dengue infections can cause serious complications, including organ failure and even in rare cases, death.

The danger of subsequent infections stems from the challenges for the human immune system in fighting a virus with four closely related serotypes: DENV-1, DENV-2, DENV-3, and DENV-4.

Researchers use the familiar concept of the Trojan horse as a way to explain why subsequent infections with dengue tend to be more serious than initial ones, Miley said.

An initial infection triggers the production of antibodies that would work well against that type of dengue. But these antibodies do not neutralize other kinds of dengue virus.

What then happens is a phenomenon that scientists call antibody-dependent enhancement (ADE). Instead of protecting against other strains, ADE allows the dengue virus easier access into immune cells and spurs more inflammation.

“You’re actually opening the door and letting it invade,” Miley said.

News accounts indicate that it may have been a repeat dengue infection that led to the death of 80-year-old Gay Small of Tampa, Florida, in September.

Her daughter, Anna Small, spoke with WFLA news, saying that her mother had traveled extensively earlier in her life. She may have had dengue previously without knowing it, Small said.

“This virus just kind of comes out of nowhere,” Small told the news station. “She met the wrong mosquito, and that’s it.”

In her younger years, Gay Small traveled with her husband to places like Asia and South America for his work, which included efforts to fight infectious diseases, Small told the Tampa Bay Times.

Small said she spoke with the media about her mother’s case to make people more aware of the risk for dengue in Florida. “If she can let people know the severity of this,” she told the Tampa Bay Times, “maybe it will give her death meaning.”

The Florida Department of Health has confirmed for media organizations, including Medscape Medical News, that there has been a first confirmed dengue-related death in Hillsborough County, Florida, where Tampa is located.

Sanofi Withdraws, Takeda Expands

There have been both notable advances and setbacks in the past decade with efforts to bring dengue vaccines to market.

Three products have been approved by regulators, with only one broadly available globally now, but not including the US.

Japan’s Takeda Pharmaceutical has secured approval of its two-dose Qdenga dengue vaccine in more than 40 countries, including India in July 2026. The European Union’s pharmaceutical regulator cleared it in 2022. Brazilian approval followed in 2023.

The US remains an exception. Takeda in 2023 voluntarily withdrew its application for FDA approval of its Qdenga vaccine, citing disagreements about “data collection.”

Takeda told Medscape Medical News that there are no immediate or additional actions planned to resume efforts to win US approval of Qdenga, but the company has continued a dialogue with the agency.

The FDA did earlier clear a different dengue vaccine for limited use in the US, but it’s no longer marketed.

The FDA in May 2019 approved Dengvaxia from France’s Sanofi. It was intended for people aged 9-16 years who lived in areas where dengue is endemic, such as Puerto Rico, who had a laboratory-documented record of a prior infection with this virus.

France’s Sanofi in 2023 decided to discontinue Dengvaxia. In an email, Sanofi told Medscape Medical News that the reason was “low global demand for this product,” and not quality, safety, or efficacy concerns.

Dengvaxia arrived at the market with high hopes. In an announcement about the 2015 Mexican approval of the vaccine, Sanofi described it as “a historic milestone for our company, for the global public health community and, most importantly, for half the world’s population who lives at risk of dengue.”

But concerns arose in the Philippines about deaths of children after Dengvaxia. Data indicated that the vaccine raised the risk for future life-threatening illness in those who had not been exposed to the virus. The government of the Philippines pulled the vaccine’s license in 2019.

The third vaccine to win a major regulatory clearance is a one-dose product, which the National Institutes of Health (NIH) helped develop with partners including Brazil’s Instituto Butantan. Brazil last year cleared this vaccine, known as Butantan-DV, an achievement that NIH described as a “quarter century of hard work” paying off.

But a Butantan-DV vaccination campaign within Brazil’s public health system was suspended in June. Brazilian health officials are examining 42 cases of adverse events, including two deaths, among people who were vaccinated.

The Brazilian Health Regulatory Agency (Agência Nacional de Vigilância Sanitária) said in June that a panel of experts would aid in the investigation of whether these reported side effects are linked to Butantan-DV.

The halt with the vaccination campaign does not mean work on the Butantan-DV vaccine had stopped in Brazil. A study of the Butantan-DV vaccine in older people (NCT06891950) continues in Brazil, a Butantan spokesperson told Medscape Medical News.

More Than 32 Million Doses

Takeda also continues to study the safety and effectiveness of Qdenga post-approval. This work includes a study designed to provide more information on its effectiveness against DEN-3 and DEN-4, which are the least common dengue virus serotypes.

There’s been fairly broad uptake of the vaccine, with Takeda saying more than 32 million doses of Qdenga have been distributed.

As published in The New England Journal of Medicine in 2019, earlier results from a pivotal study on Qdenga, known as TIDES, suggested the vaccine worked well against DENV-1 and DENV-2, while raising concerns about its effectiveness against the other two types of the virus.

So far, these hypothetical safety concerns have not emerged, said Stephen J. Thomas, MD, a professor of microbiology and immunology and public health and preventive medicine at the SUNY Upstate Medical University in Syracuse, New York. Thomas has studied dengue for more than 25 years, having earlier worked at the Walter Reed Army Institute of Research.

Thomas noted that Takeda must track and report data on how people have fared after Qdenga vaccination. Company officials must watch for reports of severe reactions and for evidence the vaccine is putting recipients at greater risk for disease or severe disease than people who are not vaccinated, he said.

“So it’s not like people aren’t looking. People are looking” for evidence of safety concerns, Thomas said. “It just hasn’t really manifested itself.”

“Is Takeda’s vaccine a home run? Absolutely not,” Thomas said. “But it does seem to be safe across different ages and different exposure histories and is delivering a public health benefit.”

Rising Concern

About 5.6 billion people now live in areas with a risk for dengue, with an estimated 100-400 million infections occurring each year, according to the World Health Organization (WHO).

This risk for dengue is likely to increase due to climate change. Expected warmer weather and other factors likely will aid the spread of the mosquitoes that transmit the virus.

For example, research suggests the ranges for dengue potentially could increase in North America, Asia, Europe, and sub-Saharan Africa to put another 2.25 billion people at risk for the infection, according to a 2022 report from the United Nations’ Intergovernmental Panel on Climate Change.

“Dengue is no longer a sporadic threat confined to the tropics. It is the mosquito-borne virus with the highest epidemic and pandemic potential,” WHO Director Tedros Adhanom Ghebreyesus, PhD, MSc, in a June statement. “Outbreaks are increasing in frequency, scale, and geographic reach, driven by urbanization, climate change, population movement, and fragile health systems.”

Miley reported no financial conflicts. Thomas reported being a consultant, advisory board member, or blinded case adjudication committee member for GlaxoSmithKline, Sanofi, Takeda, Merck, and Codagenix. He has a patent related to dengue vaccines and all rights were assigned to the US government.

Kerry Dooley Young is a journalist based in Washington, DC. She has reported on health care for more than 20 years, including in-depth coverage of the FDA and the pharmaceutical industry.