Dengue in Puerto Rico, 2010–2024: Older Patients, New Serotypes, More Hospitalizations

Dengue is a viral illness spread by mosquitoes, most often Aedes aegypti and Aedes albopictus. It can cause no symptoms at all, a nonspecific fever, or severe dengue — severe bleeding, plasma leakage or critical organ failure. With early diagnosis and adequate hydration, fewer than 1% of patients die. Cases are rising worldwide; about 13 million were reported in the Americas during 2024 by the time of this report.

Four distinct serotypes of the dengue virus (DENV-1 to DENV-4) cause illness. Infection with one gives long-lasting immunity to that serotype, but only brief protection against the other three.

In Puerto Rico, dengue is endemic, with outbreaks every 3 to 7 years. A team from CDC’s Dengue Branch, the Puerto Rico Department of Health and Ponce Health Sciences University analyzed every case reported to the department from January 1, 2010, to November 4, 2024, in three periods: 2010–2019 (the large 2010–2013 outbreaks and the quiet years after), 2020–2022 (the virus’s return and rising transmission), and 2023–2024 (the latest outbreak).

The cases

39,094 dengue cases were reported: 30,517 in 2010–2019, 2,695 in 2020–2022 and 5,882 in 2023–2024. The big outbreak years were 2010 (10,967 cases), 2012 (6,583) and 2013 (10,351). Then came an unusually quiet spell — just 365 cases in 2016–2019 — probably because the 2016 Zika outbreak left temporary cross-protective immunity. Fewer than half of patients (45.6%) were female, and 94 cases were fatal.

Older patients

In the 2010–2013 outbreaks the largest share of cases was among people aged 10–19. The median patient age was 19 in 2010–2019 and 17 in 2020–2022 — then jumped to 26 in 2023–2024. In the current outbreak 61.7% of cases are in adults 20 and older, though adolescents aged 10–19 still account for 29.5%.

Incidence rose among adults, not just their share:

Average yearly cases per 100,000 people.

The authors think changing immunity explains the shift. After the big 2010–2013 outbreaks, little dengue circulated during 2014–2019, so fewer adults built up protection; before, frequent exposure had kept adult immunity high. Bangladesh, Indonesia, Singapore and Thailand have seen the average age of dengue patients climb in the same way.

A new dominant serotype

For more than a decade DENV-1 dominated — 72.4% of typed cases in 2010–2019, and 92.9% in 2020–2022. In 2023–2024 DENV-3 took over at 45.9%, with DENV-1 down to 34.3% and DENV-2 up to 19.8%. DENV-4, 24.6% of typed cases in 2010–2019, fell to 0.1% in 2020–2022 and 0% in 2023–2024. No one was found infected with more than one serotype at once.

Little prior exposure to DENV-2 and DENV-3 may leave the population open to faster spread. DENV-3 has driven large outbreaks elsewhere recently: in Brazil it reemerged in 2023 after 15 years and caused an unprecedented outbreak in 2024; in Cuba it caused a large outbreak in 2022; and in Mexico DENV-3 cases rose in 2024.

More patients in hospital

Just over a third of patients were hospitalized in 2010–2019 (35.7%) and 2020–2022 (38.0%). In 2023–2024 it was 53.5%. The rise showed in every age group — from 40.3% to 59.0% among under-20s, 29.7% to 47.7% at 20–39, and 32.8% to 52.1% at 40 and older.

Percentage of dengue patients hospitalized, and the share of each serotype among cases with a known serotype, by period, Puerto Rico, 2010–2024. Image from CDC’s page.

Over the whole period, patients with DENV-3 were hospitalized most often (60.9%), then DENV-2 (50.2%), and DENV-1 least (37.3%). Among DENV-2 patients the rate climbed from 27.2% to 41.6% to 66.6% across the three periods.

Several things probably combined:

• DENV-2 and DENV-3 are linked to more severe disease than DENV-1.
• The health department strengthened surveillance in 2023–2024, expanding testing in commercial laboratories and interviewing every patient.
• Longer gaps between infections have been associated with more severe disease.
• Older patients are more likely to have underlying conditions.
• More severe cases may simply have been more likely to be reported.

The share of cases that were fatal did not change significantly.

Pregnancy

318 cases were in pregnant people. In 2023–2024 just over a third of them (36.4%) were hospitalized — although every pregnant person with suspected dengue should be hospitalized or closely observed, because of the risk of progressing to severe dengue.

Prevention and vaccines

Prevention means avoiding mosquito bites: repellent, long sleeves and trousers, and getting rid of containers that hold water where mosquitoes breed. The vaccine Dengvaxia protects against all four serotypes in people who have had dengue before, and is recommended for those aged 9–16 in endemic areas with evidence of past infection; it is not recommended for anyone never infected, because it raises their risk of hospitalization or severe dengue. The manufacturer is discontinuing it for lack of global demand.

Limitations

• Results were not adjusted for underlying conditions, which may raise hospitalization risk.
• People never tested or never seen by a doctor are missed, so cases are likely undercounted.
• Hospitalized patients may have been likelier to be reported, inflating the hospitalized share.
• Hospital practice may have changed, though CDC’s hospitalization recommendations for dengue did not.
• Puerto Rico’s population is aging — but dengue incidence among people 20 and older still rose.

What it means

Reemerging serotypes and lower immunity could mean more dengue hospitalizations in Puerto Rico, especially if DENV-2 and DENV-3 continue to dominate. The authors call for health care system preparedness, faster development and licensing of dengue vaccines for all ages, more training for health care providers, and wider awareness of how the disease is changing, so cases are caught early and managed promptly.

Sources

Based on Rodriguez DM, Madewell ZJ, Torres JM, et al., "Epidemiology of Dengue — Puerto Rico, 2010–2024," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.