Predominance of Den 2 and Den 3 serotypes during the 2025 dengue outbreak in Chattogram, Bangladesh: Implications for Public Health Preparedness

A 2025 study in Chattogram, Bangladesh, revealed that Dengue serotypes 2 and 3 were the predominant strains among 223 RT-PCR confirmed cases, frequently occurring in isolation or as co-infections, while serotypes 1 and 4 were rare, highlighting the need for public health strategies to address this shifting serotype landscape.

The Gist

Imagine the Dengue virus as a shape-shifting criminal gang with four distinct "identities" or costumes, known as Serotypes 1, 2, 3, and 4. In the city of Chattogram, Bangladesh, this gang has been causing trouble for years. Usually, they wear one specific costume for a while, but recently, they've started switching outfits more often.

This research paper is like a police report from early 2025 that investigates exactly which "costumes" the virus gang was wearing during a recent outbreak in Chattogram.

Here is the story of the study, broken down simply:

1. The Investigation (The Setup)

The researchers, a team of doctors and scientists from local hospitals and universities, decided to catch the virus in the act. They looked at 223 patients who came to the hospital with fever, body aches, and other signs of Dengue.

Instead of just treating the fever, they took blood samples and used a high-tech "molecular scanner" (RT-PCR) to identify exactly which version of the virus was inside each patient. They also checked the patients' ages and genders to see if the virus had a favorite target.

2. The Suspects (The Results)

When they opened their "case files," they found a clear pattern:

• The Main Culprit (Den 2): This was the star of the show. About 73% of the patients were infected with Serotype 2. It was the most common costume the virus was wearing.
• The Rising Star (Den 3): This was the second most common, found in about 7% of patients.
• The Rare Ones (Den 1 & Den 4): These were almost invisible. Only a tiny handful of patients had these versions.
• The "Double Trouble" (Co-infections): This is the most interesting part. In about 14% of cases, the patients weren't just fighting one version of the virus; they were fighting two at the same time (mostly Den 2 and Den 3 together). It's like a patient being attacked by two different criminals simultaneously.

Who got sick?
The virus didn't care much about gender; men and women were hit almost equally. However, it seemed to prefer young and middle-aged adults (roughly ages 21 to 40). Think of it like a thief who targets people who are out and about working or socializing, rather than very young children or the elderly.

3. The "Viral Load" Clue (CT Values)

The scientists also measured something called the "Cycle Threshold" (CT) value. You can think of this as a volume knob for the virus.

• A low number on the knob means the virus is screaming loudly (high amount of virus in the blood).
• A high number means the virus is whispering (low amount).

They found that the "volume" of the virus didn't change based on whether the patient was a man or a woman. However, the "volume" was different depending on which costume the virus was wearing. This tells us that different versions of the virus behave slightly differently inside the body.

4. Why Does This Matter? (The Big Picture)

The authors warn us about a dangerous game of "musical chairs."

• The Immunity Trap: If you get sick with Den 2, your body builds a shield against Den 2. But that shield does nothing against Den 3.
• The Double Whammy: Because Den 2 and Den 3 are now circulating together, a person who survived a Den 2 infection last year might get hit by Den 3 this year. This "second hit" is often much more dangerous and can lead to severe illness or death. It's like the immune system gets confused and accidentally helps the new virus attack the body harder.

5. The Takeaway

The study concludes that the virus in Chattogram is changing its strategy. It's no longer just one type; it's a mix of Den 2 and Den 3, sometimes even attacking in pairs.

What should we do?

• Mosquito Control: We need to smash the mosquito breeding grounds (standing water) because the mosquitoes are the delivery drivers for these viruses.
• Vaccine Hope: The authors say we desperately need a "universal vaccine" (a tetravalent vaccine) that trains our immune system to recognize all four costumes at once, so we aren't caught off guard when the virus switches outfits.
• Public Awareness: Doctors and the public need to be ready for these "double infections" and understand that the rules of the game are changing.

In short: The Dengue virus in Chattogram is evolving. It's mostly wearing the "Den 2" mask, but it's increasingly teaming up with "Den 3." This mix makes the situation more complex and potentially more dangerous, requiring us to be smarter about prevention and treatment.

Technical Summary

1. Problem Statement

Dengue fever remains a critical public health challenge in Bangladesh, with the 2023 outbreak being the deadliest in the country's history. By 2025, transmission had expanded to become a year-round threat across all 64 districts, with over 105,000 cases recorded despite a slight decrease in mortality compared to 2024.

• The Core Issue: The epidemiology of dengue is complicated by the existence of four distinct serotypes (DENV-1 to DENV-4). Infection with one serotype provides lifelong immunity to that specific type but not others. Secondary infections with heterotypic serotypes carry a high risk of severe disease due to Antibody-Dependent Enhancement (ADE).
• Knowledge Gap: While DENV-2 had been the dominant serotype in Bangladesh for several years, there was a lack of recent, localized data regarding serotype shifts, co-infections, and viral load dynamics specifically in Chattogram for the 2025 outbreak. Understanding these shifts is vital for predicting outbreak severity and preparing public health responses.

2. Methodology

The study was a cross-sectional analysis conducted at Chattogram Maa O Shishu Hospital Medical College (CMOSHMC) between July 1, 2025, and December 31, 2025.

• Study Population:
  • Sample Size: 223 RT-PCR confirmed dengue-positive patients.
  • Inclusion Criteria: Patients aged 5–65 years presenting with clinical symptoms (fever onset 2–5 days, rash, myalgia, bone pain, etc.).
  • Exclusion Criteria: Severely ill patients or those with acute conditions where invasive sampling was contraindicated.
• Laboratory Procedures:
  1. Screening: Initial screening via Dengue NS1 Rapid Diagnostic Test (RDT).
  2. Sample Collection: 3mL venous blood collected in Serum Separating Tubes (SST); serum separated and cryopreserved at –80°C.
  3. RNA Extraction: Performed using the CE IVDR Marked GeneProof RNA Extraction Kit.
  4. Serotyping: Real-time RT-PCR using the RUO Marked NeoDx Dengue Genotyping Real-Time PCR Kit (India) to detect DENV-1 through DENV-4.
  5. Thresholds: Cycle Threshold (Ct) values <38 were considered positive; >38 were negative.
• Statistical Analysis:
  • Data analyzed using Microsoft Excel and GraphPad Prism 10.4.
  • Comparisons: Mann-Whitney U test was used to compare median Ct values between sexes and between different serotypes (due to non-normal distribution).
  • Visualization: Violin plots were generated to display Ct value distributions.

3. Key Contributions

• First Localized Serotype Profile for 2025: Provides the first detailed breakdown of circulating dengue serotypes in Chattogram during the 2025 outbreak, highlighting a significant shift in the viral landscape.
• Quantification of Co-infections: Systematically documented the prevalence of concurrent infections (mixed serotypes), a critical factor often overlooked in routine surveillance but crucial for assessing severe disease risk.
• Viral Load Analysis: Correlated serotype types with Cycle Threshold (Ct) values (a proxy for viral load) and analyzed sex-based differences in viral burden.
• Public Health Alert: Identifies the emergence of DENV-3 alongside DENV-2, signaling a potential "serotype twisting" event that could increase the risk of severe dengue via ADE mechanisms.

4. Key Results

• Demographics:
  • Gender: Nearly equal distribution (Males: 113, Females: 110).
  • Age: Predominantly middle-aged adults. The mean age was 33.55 ± 13.67 years. The highest prevalence was in the 21–30 age group (31.4%), followed by 31–40 (24.7%).
• Serotype Distribution:
  • DENV-2: The dominant serotype, accounting for 73.1% (n=163) of isolated cases.
  • DENV-3: The second most prevalent, at 6.7% (n=15) of isolated cases.
  • DENV-1: Rare, found in only 1.3% (n=3) of isolated cases.
  • DENV-4: Not found as an isolated case; only detected in co-infections.
• Co-infections (Mixed Serotypes):
  • DENV-2 + DENV-3: The most common co-infection, observed in 14.3% (n=32) of patients.
  • DENV-1 + DENV-2: Observed in 3.6% (n=8).
  • Triple Infection (DENV-1, 2, 3) & DENV-3 + DENV-4: Extremely rare, each occurring in 0.4% (n=1).
• Viral Load (Ct Values):
  • Sex Differences: No statistically significant difference in Ct values between males and females for DENV-2 or DENV-3.
  • Serotype Differences: Significant variations in Ct values were observed when comparing DENV-1 against DENV-2 and DENV-3 (p < 0.05). However, the difference in Ct values between DENV-2 and DENV-3 was not statistically significant.

5. Significance and Implications

• Risk of Severe Disease: The high prevalence of DENV-2 combined with the significant rise in DENV-3 and co-infections (14.3%) creates a high-risk environment for severe dengue. The co-circulation of multiple serotypes increases the likelihood of secondary heterotypic infections, triggering ADE and potentially leading to higher morbidity and mortality rates.
• Surveillance Shift: The data indicates a shift from the previous dominance of DENV-2 alone to a scenario where DENV-2 and DENV-3 are co-dominant. This "serotype twisting" necessitates updated surveillance protocols to track DENV-3 more closely.
• Public Health Preparedness:
  • Vector Control: Urgent need for intensified mosquito control measures, as the disease has become a year-round threat.
  • Clinical Awareness: Healthcare providers must be trained to recognize the potential for severe outcomes in patients with mixed serotype infections.
  • Vaccine Strategy: The findings underscore the critical need for a tetravalent vaccine capable of protecting against all four serotypes to prevent immune-enhanced severity.
• Limitations: The study did not correlate serotype data with clinical outcomes (severity/death), which remains a limitation for fully assessing the immediate impact of these serotype shifts on patient mortality.

Conclusion: The 2025 dengue outbreak in Chattogram is characterized by the predominance of DENV-2 and a significant resurgence of DENV-3, with a notable rate of co-infection. This epidemiological shift poses a serious threat to public health, requiring immediate strategic interventions in vector control, clinical management, and vaccine development.