Dengue in India: DENV Serotypes, Aedes aegypti, Qdenga & 2024-25 Outbreaks

!Aedes aegypti mosquito, the principal dengue vector

Dengue in India has shifted, in the space of two decades, from an episodic monsoon nuisance to a year-round urban emergency. The Ministry of Health and Family Welfare reports roughly 290,000 confirmed dengue cases and around 485 dengue-related deaths in 2024 — the highest annual case count India has recorded — and the National Centre for Vector Borne Diseases Control (NCVBDC) acknowledges that the true burden, after correcting for under-reporting from the private sector, is likely several times higher. Dengue is now reported from all 36 states and union territories, and the geographical heart of the disease has moved decisively from the Eastern coast to the entire Indo-Gangetic plain, the Deccan and the metropolitan agglomerations.

Dengue in India is caused by four antigenically distinct serotypes of the dengue virus and transmitted by the Aedes aegypti mosquito. The peculiar epidemiology of dengue — sequential infection by different serotypes can produce severe disease via antibody-dependent enhancement — has shaped both vaccine development and India’s vector-control strategy. The current focus is on integrated mosquito source-reduction, ICMR’s serotype surveillance, the rollout of the Qdenga (TAK-003) vaccine by Takeda since 2022 and the indigenous tetravalent vaccine being developed by Panacea Biotec with the NIAID.

The Dengue Virus and Serotypes

Dengue is caused by dengue virus (DENV), a single-stranded RNA flavivirus closely related to the viruses of yellow fever, Japanese encephalitis, West Nile and Zika. Four serotypes — DENV-1, DENV-2, DENV-3 and DENV-4 — circulate, each capable of causing the full clinical spectrum.

Primary and Secondary Infection

Infection by one serotype produces lifelong protective immunity against that serotype but only short-lived (around three months) cross-protection against the others. A subsequent infection by a different serotype carries a substantially higher risk of severe disease. The mechanism is antibody-dependent enhancement: non-neutralising antibodies from the first infection bind to the new virus and facilitate its entry into immune cells, generating a hyperinflammatory response. This is why dengue tends to become more severe with each successive epidemic wave in a previously dengue-naive population.

All Four Serotypes in India

All four DENV serotypes circulate in India simultaneously, and the dominant serotype varies by state and year. DENV-2 was historically the most common; DENV-1 dominated in 2017-19; the 2023 and 2024 outbreaks in north India were driven largely by DENV-2 and DENV-3. ICMR’s National Institute of Virology (NIV) in Pune runs the country’s apex serotype surveillance through the Virus Research and Diagnostic Laboratories network of more than 160 sites.

Clinical Spectrum

The 2009 WHO case classification, adopted by India, divides dengue into three categories.

Dengue Fever

Classical dengue fever begins three to fourteen days after the infectious mosquito bite and presents with high fever, severe headache, retro-orbital pain, myalgia (“breakbone fever”), arthralgia and a maculopapular rash. Most cases resolve in five to seven days with supportive care.

Dengue with Warning Signs

Warning signs appear at the end of the febrile phase (days three to seven) and herald plasma leakage. They include severe abdominal pain, persistent vomiting, mucosal bleeding, restlessness, hepatomegaly more than 2 cm and a sudden drop in platelet count with a rising haematocrit. Patients with warning signs require hospitalisation and close monitoring.

Severe Dengue

Severe dengue — formerly dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) — is defined by one of three criteria: severe plasma leakage producing shock or respiratory distress, severe bleeding, or severe organ involvement (transaminases above 1,000, altered consciousness, myocarditis). Severe dengue carries a case-fatality rate of around 1 to 5 per cent without intensive care and below 1 per cent with timely fluid resuscitation. India’s clinical management guidelines, published by NCVBDC and revised in 2023, emphasise judicious crystalloid use and avoidance of aspirin and NSAIDs.

The Aedes aegypti Vector

Dengue is transmitted almost exclusively by Aedes aegypti, an originally African mosquito that has spread across the tropics with human commerce. Aedes albopictus, the Asian tiger mosquito, is a secondary vector in southern and eastern India.

Aedes aegypti Biology

Aedes aegypti is a day-biting mosquito, most active in early morning and late afternoon, that lives in close association with humans. It breeds in clean stored fresh water — overhead tanks, drums, flowerpots, coolers, discarded tyres, plastic containers, AC trays and construction-site puddles. Eggs can withstand desiccation for up to a year, hatching the moment water is added. The mosquito has a flight range of only about 100 to 200 metres, which means dengue outbreaks are intensely focal and source reduction within a household compound is the single most effective intervention.

Urban Breeding Sites

The shift of Aedes aegypti to year-round breeding in Indian cities is driven by intermittent water supply (encouraging household water storage), unplanned construction, single-use plastic litter and rising temperatures. The arc of dengue cases now extends through the post-monsoon months of October-November well into December and even January in southern India, contradicting the older textbook description of dengue as a strictly monsoonal disease.

ICMR-NIV and Surveillance

ICMR’s National Institute of Virology, Pune (ICMR-NIV) is the apex national reference laboratory for dengue. NIV runs the NS1 antigen and IgM ELISA-based surveillance through the network of more than 160 Virus Research and Diagnostic Laboratories (VRDLs) and the Apex Referral Laboratories network. The Integrated Disease Surveillance Programme (IDSP) collects weekly dengue case data from all states, and the NCVBDC issues weekly bulletins. Serotype monitoring is critical because the introduction of a new dominant serotype into an immunologically primed population precipitates the most severe outbreaks.

Vaccines for Dengue

Dengue vaccination has been complicated by the requirement that any vaccine must protect against all four serotypes simultaneously to avoid increasing the risk of severe disease in seronegative individuals.

Dengvaxia (CYD-TDV)

The first dengue vaccine — Dengvaxia, by Sanofi Pasteur — was licensed in 2015 but is now restricted by WHO to people who are dengue-seropositive (i.e. who have had at least one previous dengue infection), because of evidence that seronegative recipients face an increased risk of severe dengue. India has not approved Dengvaxia.

Qdenga (TAK-003)

Qdenga, developed by Takeda Pharmaceuticals, is a live attenuated tetravalent dengue vaccine based on a DENV-2 backbone. It received European Medicines Agency approval in December 2022 and WHO prequalification was granted in May 2024, paving the way for procurement by national programmes. Qdenga is administered as a two-dose schedule three months apart. Pivotal Phase III trial data showed roughly 80 per cent efficacy against virologically confirmed dengue and around 90 per cent against hospitalised dengue over the first year. Qdenga is not yet part of India’s Universal Immunisation Programme; the Government has commissioned cost-effectiveness analyses and is reviewing the case for selective introduction.

Panacea-NIAID Tetravalent Vaccine

Panacea Biotec, in partnership with the US National Institute of Allergy and Infectious Diseases (NIAID), is developing an indigenous live-attenuated tetravalent dengue vaccine (TV003/TV005 backbone). The Phase III trial across 19 sites in India is the largest-ever vaccine trial in the country with more than 10,000 participants. Trial enrolment was completed in 2024 and topline results are expected in 2026-27. If successful, it would be the first dengue vaccine designed for, and manufactured in, India at a price suitable for public-sector rollout.

2024-25 Outbreaks

The 2024 dengue season was India’s largest on record. Bengaluru reported more than 25,000 confirmed cases by October — an entomological emergency declared by the Karnataka government — and Pune crossed 15,000. North India’s heatwave-extended monsoon prolonged the breeding season; Delhi, Lucknow, Patna and Kolkata all reported substantial spikes between September and November. The Kerala outbreak of mid-2024 was driven mainly by DENV-2 and DENV-3 co-circulation. Pre-monsoon dengue in 2025 has already been reported from Mumbai, Bengaluru and Hyderabad, suggesting further shift to year-round transmission. The intersection of dengue with the broader public health architecture and the one-health approach — recognising the linkages between mosquitoes, urban ecology and climate — has become central to policy.

ABER and Entomological Indicators

Entomological surveillance is the backbone of dengue prevention. The four standard indicators, taught in every public-health curriculum, are:

Aedes Breteau Index (BI)

The Breteau Index is the number of positive larval containers per 100 houses inspected. A BI above 20 indicates high dengue-transmission risk; the elimination threshold is below 5.

House Index (HI)

The House Index is the percentage of houses with at least one positive container. An HI below 1 per cent is the recommended threshold for low risk.

Container Index (CI)

The Container Index is the percentage of water-holding containers positive for Aedes larvae. It is sensitive to the focal nature of urban breeding.

Aedes Breeding Eradication Rate (ABER)

The Aedes Breeding Eradication Rate (or Aedes-Breteau Eradication Rate, sometimes also recorded as the “ABER” entomological coverage index) measures the proportion of houses successfully inspected and treated by the vector-control team out of those targeted. A coverage above 10 per cent of total households per round is the standard benchmark. NCVBDC also tracks the Pupal Index in priority cities.

Vector Control and Source Reduction

The principal interventions are source reduction (community-led “dry day” once a week to empty all water-storage containers), peri-focal larviciding with temephos in unavoidable storage and indoor space spraying with synthetic pyrethroids during outbreaks. Wolbachia-infected Aedes aegypti releases — pioneered by the World Mosquito Program in Australia, Indonesia and Brazil — are now being piloted by ICMR-VCRC (Vector Control Research Centre, Puducherry) in selected Indian sites since 2023. The Wolbachia strategy works by reducing the ability of Aedes aegypti to transmit dengue and is one of the most promising biological interventions of the decade.

Dengue in India for UPSC

Dengue in India is a recurring topic in UPSC General Studies Paper II (health) and Paper III (science and technology). Candidates should know that all four DENV serotypes circulate in India simultaneously, that Aedes aegypti is a day-biting urban vector that breeds in clean stored fresh water, that ICMR-NIV Pune is the apex reference laboratory, that Qdenga (TAK-003) received WHO prequalification in May 2024, that the Panacea-NIAID indigenous tetravalent vaccine is in Phase III in India with results expected in 2026-27, and that the principal entomological indicators are Breteau Index, House Index, Container Index and ABER.

Frequently Asked Questions