Puerto Rico's 2024 Dengue Outbreak and the Response

By Fhallon Ware-Gilmore, Dania M. Rodriguez, Kyle Ryff, Jomil M. Torres, Miladys Perez Velez, Cristhian T. Torres-Toro, Gilberto A. Santiago, Aidsa Rivera, Zachary J. Madewell, Yashira Maldonado, Iris Cardona-Gerena, Grayson C. Brown, Laura E. Adams, Gabriela Paz-Bailey and Melissa Marzán-Rodriguez — CDC, the Puerto Rico Department of Health and the Puerto Rico Vector Control Unit

At a glance

• Known: dengue, spread by mosquitoes, is endemic in the tropics and subtropics and can cause severe illness or death. Puerto Rico's last outbreak was in 2013.
• New: in 2024, Puerto Rico reported 6,291 cases and passed the epidemic threshold, and declared a public health emergency. About half the patients (52.3%) were hospitalized, 264 (4.2%) had severe dengue, and 11 (0.2%) died. People aged 10–19 accounted for 28.4% of severe cases.
• So what: recognizing cases and managing them well improves outcomes. Residents and visitors should use repellent, wear protective clothing, and stay where doors and windows are screened.

Dengue

Dengue is a viral infection spread mainly by Aedes aegypti mosquitoes. Four serotypes (DENV-1 to DENV-4) cause it, and it ranges from no symptoms to severe disease (1).

• Symptoms usually start 3–10 days after exposure: fever, muscle and joint pain, pain behind the eyes, rash, nausea and vomiting.
• No antiviral treats it. Spotting the warning signs of severe dengue early, proper triage, supportive care and follow-up are what reduce illness and death (2).
• The vaccine: Dengvaxia is recommended for children 9–16 living in U.S. areas where dengue is endemic who have had a confirmed past infection; it protects against symptomatic disease and hospitalization.

About 13 million dengue cases were reported to the Pan American Health Organization from the Americas in 2024 (3). In Puerto Rico, transmission peaks July–November, with outbreaks every 3–7 years (4). The 11-year gap before 2024 was unusually long — likely because of temporary cross-protection after the 2016 Zika outbreak, and less travel and exposure during the COVID-19 pandemic.

The emergency

In January 2024, cases passed the epidemic threshold — the weekly level historically tied to epidemics, based on 30 years of surveillance — and stayed there. In March 2024, the Puerto Rico Department of Health (PRDH) declared a public health emergency (5,6) and, with CDC, launched a response: tracking the outbreak, controlling mosquitoes, and campaigns on preventing bites.

Weekly dengue cases in 2024 (solid), the epidemic threshold (dashed) and the historical median since 1986 (dotted). CDC.

The outbreak in numbers

6,291 confirmed cases — 191.4 per 100,000 people — stayed above the threshold all year, rising after the rainy season began in May. The most cases came in October (884), with a weekly peak of 226 in week 40; transmission was still above the threshold in January 2025.

Where: the most cases were in the San Juan area, but the highest rates were in smaller municipalities in the center and west:

Dengue cases (A) and incidence per 100,000 (B) by municipality, 2024. CDC.

Who (of 6,291 cases):

• 53.5% of cases were in males; the median age was 24 (2.3 months to 99 years).
• Teenagers (10–19) had the most cases (29.3%), hospitalizations (33.9%) and severe cases (28.4%). Severe dengue means life-threatening shock, bleeding or organ failure.
• Deaths fell mostly on older adults: four among those 70 or older, and two each at 50–59 and 60–69.

Which virus: of 4,942 cases confirmed by RT-PCR, DENV-3 caused 2,926 (59.2%) and DENV-1 1,367 (27.7%). The three serotypes circulating (DENV-1, -2 and -3) were new variants, introduced to the island in 2019–2023.

The response

Surveillance and testing

• A new, integrated case database, linking data from commercial laboratories.
• Broader testing through commercial labs, and a free dengue testing center (RT-PCR and IgM) for patients with a medical order.
• Interviews with every confirmed patient about exposure and symptoms.
• Weekly coordination between PRDH, CDC, the Puerto Rico Vector Control Unit (PRVCU) and partners.

Training: a mandatory dengue course for medical professionals, created in October 2023, was completed by about 10,800 providers by November 2024. PRDH and CDC held 3,030 education and prevention events, reaching about 167,000 people across all eight health regions.

Mosquitoes

• January–November 2024: 234,713 Ae. aegypti caught in autocidal gravid ovitraps; of 28,932 mosquito pools tested, 316 (1.1%) carried dengue virus — most in San Juan (250 of 7,985). Positive pools were 51% DENV-3, 41% DENV-2 and 8% DENV-1.
• Control: wide-area larviciding, mass trapping, space and residual sprays, and yard inspections to remove breeding sites; weekly sanitation surveys; guidance from the U.S. Department of Agriculture on pesticide use where cases persisted or mosquitoes resisted insecticides.
• September 2024: an Integrated Vector Management advisory committee, recommending rotating and regularly testing insecticides to limit resistance.

Public outreach — about 160,000 people reached by December 2024:

• 215 activities in high-risk communities and 328 at schools;
• 1,624 health fairs and community events, and visits to 108 health care facilities;
• 56,809 educational kits;
• the campaign "Haz la diferencia y ciérrale la puerta al dengue" ("Make a difference and close the door to dengue") on social media and radio: empty standing water, use repellent, and screen doors and windows.

Vaccination: prevaccination screening in three lab networks, and clinics offering Dengvaxia grew from six to 23 in 2024. But with low global demand, the maker has discontinued Dengvaxia; remaining doses expire in 2026, and no other dengue vaccine is approved in the United States.

What it means

• An unusual outbreak: it began in the low season and grew into the high season, peaking in October and staying high through December — raising concern about heavy transmission into the next season, and possibly shifting patterns as population immunity changes.
• Hospitalizations were high, perhaps because DENV-3 (58.7% of hospitalizations with a known serotype) is linked to more severe disease (8) — though better tracking of outcomes since 2023, including interviews with every suspected case, may also play a part.
• Priorities: children and teenagers need focused prevention; older adults need clinical resources. The highest rates were outside the metro area.
• Regionally: the Americas saw record case numbers in 2024 (9,10). Safe, effective vaccines for all ages and scalable mosquito control are urgently needed.

If you live in or visit Puerto Rico: use EPA-approved repellent, wear protective clothing, and stay where doors and windows have screens. Clinicians should suspect dengue in anyone with fever and possible mosquito exposure there.

Sources

Based on Fhallon Ware-Gilmore, Dania M. Rodriguez, Kyle Ryff, Jomil M. Torres, Miladys Perez Velez, Cristhian T. Torres-Toro, Gilberto A. Santiago, Aidsa Rivera, Zachary J. Madewell, Yashira Maldonado, Iris Cardona-Gerena, Grayson C. Brown, Laura E. Adams, Gabriela Paz-Bailey and Melissa Marzán-Rodriguez, "Dengue Outbreak and Response — Puerto Rico, 2024," MMWR, Centers for Disease Control and Prevention; a work of the United States government in the public domain. Carolina's count is given as 354, as in the report's abstract; its results section says 254, which does not match the 5.1% beside it. Numbers in parentheses refer to the report's references. Its social media graphic is not reproduced.