Has a Natural Endemic Focus for Dengue Been Established in Fujian Province,China? An Assessment Based on Four Core Evidence Dimensions, 2014-2024

Based on a comprehensive 11-year assessment (2014–2024) utilizing four core evidence dimensions, this study concludes that Fujian Province, China, has not established a stable natural endemic focus for dengue fever, as local transmission remains entirely dependent on imported cases, evidenced by the complete interruption of local transmission during the pandemic, low population seroprevalence, minimal viral detection in vectors, and an adult-skewed age distribution.

The Gist

🌴 The Big Question: Is Dengue "Moving In" to Fujian?

Imagine Fujian Province (a coastal area in China) as a garden.

• The Garden: It has the perfect soil, rain, and temperature for mosquitoes (the "weeds") to grow.
• The Problem: Dengue fever is a disease carried by these mosquitoes.
• The Fear: For over a decade, people saw dengue cases appearing in this garden every year. The big question was: "Has the disease taken root and started growing on its own, or is it just a visitor that keeps showing up?"

In scientific terms, researchers wanted to know if Fujian had established a "Natural Endemic Focus."

• Endemic Focus: Think of this as a permanent resident. The virus lives there, spreads from person to person, and doesn't need anyone to bring it in from outside.
• Imported Cases: Think of this as a tourist. The virus arrives with a traveler from another country, causes a few cases, and then dies out unless another tourist brings it back.

The researchers spent 11 years (2014–2024) investigating this garden to see if the virus had become a permanent resident.

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🕵️‍♂️ The Investigation: Four Clues

To solve this mystery, the team looked at four specific clues, like detectives at a crime scene. They also used a unique "natural experiment": The COVID-19 Pandemic.

Clue 1: The "Traffic Stop" (The Pandemic Experiment)

• The Analogy: Imagine a busy highway leading into the garden. Usually, cars (travelers) bring dengue in.
• What Happened: During the COVID-19 pandemic (2020–2022), the government put up a massive roadblock. No one was allowed to travel internationally.
• The Result: The "tourists" stopped coming. And guess what? The dengue cases in Fujian stopped completely.
• The Lesson: If the disease was a permanent resident living in the garden, it would have kept spreading even without new tourists. The fact that it vanished proves the virus cannot survive on its own in Fujian; it needs a constant supply of new visitors to keep the fire burning.

Clue 2: The "Memory Bank" (Blood Tests)

• The Analogy: When people get sick with a virus, their bodies build a "memory" (antibodies) to fight it later. In a place where a disease is common (endemic), almost everyone has this memory bank filled up.
• The Result: The researchers tested over 15,000 healthy people in Fujian. Only 4.2% had this "memory."
• The Lesson: In a true dengue country (like parts of Southeast Asia), 50% to 70% of people would have this memory. In Fujian, the population is mostly "naive" (hasn't seen the virus before). This means the virus isn't circulating quietly in the background.

Clue 3: The "Mosquito Check" (Vector Surveillance)

• The Analogy: Mosquitoes are the delivery trucks for the virus. If the virus is a permanent resident, the trucks should be loaded with it all the time.
• The Result: The team caught and tested 385,000 mosquitoes over 11 years. They found the virus in only one single mosquito (in 2017).
• The Lesson: The delivery trucks are almost always empty. The virus isn't hiding in the mosquito population waiting to jump to the next person.

Clue 4: The "Who Got Sick?" (Age Patterns)

• The Analogy: In a place where a disease is permanent, kids get sick the most because they haven't built up immunity yet. It's like a school where the flu is always going around.
• The Result: In Fujian, the people getting sick were mostly middle-aged adults (40–60 years old). Very few children were affected.
• The Lesson: This pattern suggests the virus is only reaching people who travel or work in high-risk areas (adults), not spreading through the whole community like a wildfire in a school.

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🏁 The Verdict

The garden is not infested.

The study concludes that Fujian Province has NOT established a stable, natural home for dengue fever.

• The virus is a tourist, not a resident.
• Every local outbreak happens because someone brought the virus in from abroad.
• Once the "tourists" stop coming (like during the pandemic), the local transmission stops immediately.

🛡️ What Should We Do? (The Action Plan)

Even though dengue isn't a permanent resident yet, Fujian is still at high risk because the "weather" (climate) is perfect for mosquitoes, and the "highway" (travel) is open again.

The researchers suggest three strategies to keep the virus from ever moving in permanently:

1. Guard the Gate (Surveillance): Be super strict at airports and borders. If a traveler comes back sick, catch them immediately so they don't infect local mosquitoes.
2. Mow the Lawn (Mosquito Control): During the rainy, hot months (June–October), aggressively kill mosquito breeding spots. If you stop the "delivery trucks" (mosquitoes), the virus can't get in.
3. Educate the Neighbors (Public Awareness): Teach people, especially travelers and adults, to recognize symptoms and protect themselves from bites.

💡 The Bottom Line

Fujian is currently a high-risk zone that is successfully keeping dengue out, but it needs to stay vigilant. If they stop watching the borders or stop controlling mosquitoes, that "tourist" virus could eventually decide to move in permanently, turning Fujian into a place where dengue is always present. The good news? We caught it in time, and we know exactly how to keep it out.

Technical Summary

1. Problem Statement

Dengue fever is a rapidly escalating global health burden. While historically imported into southern China, the region faces increasing risks of local transmission due to favorable ecological conditions for Aedes albopictus (the primary vector) and frequent cross-border travel with endemic Southeast Asian countries.

• The Core Question: Has Fujian Province transitioned from a region of sporadic, imported-driven outbreaks to a stable natural endemic focus (where the virus circulates sustainably within the local population independent of imported sources)?
• The Gap: While local cases have been reported annually in Fujian for over a decade, no systematic assessment using long-term, multi-dimensional surveillance data has been conducted to determine if the province meets the four core epidemiological criteria for endemicity.

2. Methodology

The study employed a comprehensive, 11-year retrospective analysis (2014–2024) utilizing a four-dimensional evidence framework to assess endemicity. The study period was stratified into three phases to leverage the "natural experiment" of the COVID-19 pandemic:

1. Pre-pandemic (2014–2019): Normal transmission dynamics.
2. Pandemic (2020–2022): Strict non-pharmacological interventions (NPIs) drastically reduced cross-border travel.
3. Post-pandemic (2023–2024): Recovery of travel and transmission.

Data Sources & Collection:

• Case Data: Extracted from China's National Notifiable Disease Reporting System (NNDRS) for 3,606 cases (1,229 imported, 2,377 locally acquired).
• Serological Surveillance: 15,736 serum samples from healthy residents at seven sentinel sites were tested for Dengue IgG antibodies (ELISA) during pre- and post-epidemic seasons.
• Vector Surveillance: Routine collection and testing of 385,000 Aedes albopictus mosquitoes for Dengue virus (DENV) nucleic acid (RT-PCR) at the same seven sites.
• Demographic Analysis: Age distribution of locally acquired vs. imported cases.

Statistical Analysis:

• Descriptive epidemiology (temporal and spatial distribution).
• Statistical comparisons using t-tests (continuous variables like age) and Pearson's $\chi^2$ tests (categorical variables like seropositivity).
• Software: R (v4.4.1) and ArcGIS 10.2.

3. Key Contributions

• First Systematic Assessment: This is the first study to apply the four internationally recognized criteria for dengue endemicity specifically to Fujian Province using a decade of integrated surveillance data.
• Natural Experiment Validation: The study uniquely utilizes the COVID-19 pandemic as a control variable. The complete cessation of local transmission during strict travel restrictions provided definitive causal evidence regarding the dependency of local transmission on imported cases.
• Multi-Dimensional Integration: It synthesizes epidemiological, serological, virological, and demographic data, moving beyond simple case counting to assess the sustainability of transmission.
• Methodological Framework: It provides a replicable model for assessing dengue endemic potential in other subtropical coastal regions facing similar imported virus threats.

4. Key Results

The study concluded that Fujian Province has NOT established a stable natural endemic focus. Evidence across all four dimensions supports this:

• Dimension 1: Temporal Continuity (Transmission Interruption)

  • Pandemic Phase (2020–2022): Local transmission was completely interrupted (0 local cases) when imported cases dropped to near zero (avg. 9/year).
  • Post-Pandemic: Local cases (avg. 80/year) only resumed when cross-border travel and imported cases recovered. This confirms local transmission is entirely dependent on imported viral sources.

• Dimension 2: Serological Profile (Population Immunity)

  • Low Seroprevalence: The overall population IgG positivity rate was only 4.2% (66/15,736).
  • No Seasonal Shift: No statistically significant difference between pre-epidemic (3.8%) and post-epidemic (4.5%) seasons ($P=0.48$).
  • Threshold: No year exceeded a 10% positivity rate, far below the >50% typically seen in endemic regions (e.g., Philippines at ~76%).

• Dimension 3: Vector Virology (Viral Circulation)

  • Rare Detection: Out of 385,000 mosquitoes tested over 11 years, only one sample was DENV-positive (Fuzhou, Oct 2017), linked directly to a known local outbreak that year.
  • No Reservoir: The absence of viral detection in the remaining samples confirms no sustained viral circulation or viral reservoir exists within the local mosquito population.

• Dimension 4: Age Distribution (Epidemiological Pattern)

  • Adult-Dominant: Locally acquired cases had a significantly higher mean age (46.1 years) compared to imported cases (35.8 years).
  • No Child-Dominance: Unlike endemic regions where children (naive immunity) dominate the case load, Fujian's local cases were concentrated in the 40–60 age group. This indicates transmission is limited to adults with higher exposure to imported sources, rather than permeating the broader community.

5. Significance and Implications

• Public Health Status: Fujian remains a non-endemic region where local outbreaks are strictly driven by imported cases. The virus has not established a self-sustaining cycle.
• Risk Assessment: Despite the lack of endemicity, the risk of future outbreaks remains high due to:
  • Subtropical climate ideal for Aedes breeding.
  • Continuous influx of imported cases from Southeast Asia.
  • Potential climate change impacts (warming expanding vector range).
• Strategic Recommendations: To prevent the establishment of an endemic focus, the authors propose three evidence-based strategies:
  1. Enhanced Surveillance: Strengthen port health quarantine and early detection of imported cases to block the primary viral entry route.
  2. Targeted Vector Control: Implement integrated mosquito control (source reduction, larvicides) specifically during the peak transmission season (June–October).
  3. Public Education: Target high-risk groups (travelers, healthcare workers) to improve symptom recognition and reduce exposure.

Conclusion: The study definitively demonstrates that while Fujian is vulnerable to imported-driven outbreaks, it has not crossed the threshold into a natural endemic focus. Continued vigilance and targeted interventions are critical to maintaining this status and preventing the virus from becoming permanently established in the region.