India's First Dengue Strain Map Reveals Hyperendemicity

India’s first nationwide molecular surveillance of dengue viruses has uncovered that all four serotypes are circulating simultaneously in eight states and Union Territories, making the country increasingly hyperendemic. The study, led by the ICMR-National Institute of Virology (NIV) in Pune, analysed 6,889 laboratory-confirmed dengue samples collected through a network of 45 Virus Research and Diagnostic Laboratories across 25 states and Union Territories between 2023 and 2025. The findings come as the country prepares to roll out its first dengue vaccine, expected as early as next year.

Across the country, DENV-2 remained the dominant strain, accounting for 51% of infections, followed by DENV-3 at 13%, DENV-1 at 7% and DENV-4 at just 1%. However, significant regional variation emerged: DENV-1 was responsible for a larger share of cases in Tamil Nadu, Haryana and Rajasthan, while DENV-3 featured prominently in Gujarat, Uttar Pradesh and West Bengal. Perhaps most striking, 7% of patients were infected with two or more serotypes at the same time, and these co-infections were associated with more severe clinical outcomes—including thrombocytopenia, haemorrhagic manifestations and joint pain.

With a national dengue vaccine deployment on the horizon, researchers say the data offers an essential blueprint. Because the efficacy of available vaccine candidates can differ by serotype, knowing exactly which strains are dominant in which regions will shape where and how the immunisation programme is rolled out. The study underscores the need for ongoing molecular surveillance to adapt vaccination strategies as virus circulation patterns evolve.

What the Serotype Data Means for Vaccine Strategy and Severe Disease Burden

Why a National Strain Map Matters for the Upcoming Vaccine Rollout

The detection of all four serotypes across large, populous states like Maharashtra, Tamil Nadu and West Bengal means that a tetravalent vaccine—one that protects against all four—will be crucial. If the national programme seeds vaccines where only a single strain is prevalent, the protection may be incomplete, and the risk of severe disease from other serotypes could remain. Health authorities can now use this map to prioritise states where simultaneous circulation is highest, potentially preventing the most severe outcomes.

Co-circulation and the Risk of Severe Dengue

The 7% co-infection figure is a clear warning sign. A subsequent infection with a different serotype is known to increase the likelihood of severe dengue through antibody-dependent enhancement, and simultaneous infections appear to compound that risk. Clinicians and hospital systems in hyperendemic states may need to prepare for a higher proportion of complex cases, and public awareness campaigns should stress early care-seeking when symptoms appear.

Regional Variations Demand State-Level Planning

While DENV-2 dominates nationally, the regional data show that a one-size-fits-all vaccination approach could miss the mark. For example, a vaccine that performs well against DENV-2 but is less effective against DENV-1 might offer suboptimal protection in Haryana or Tamil Nadu, where DENV-1 is a major contributor. State immunisation officials will need to align vaccine selection and deployment schedules with local serotype prevalence, a task that continuous surveillance can support.

The Urgent Need for Sustained Surveillance

The study is a snapshot of three years, but dengue serotype dominance can shift seasonally and year-to-year. A robust, real-time molecular surveillance network—building on the 45 VRDLs already in use—will be essential not only for forecasting outbreaks but also for updating the vaccine strategy if a new serotype gains ground. Without this, India risks deploying a vaccine that rapidly becomes mismatched to the circulating strains.

Preparing for the Rollout: What Health Authorities and the Public Need to Know

For public health officials and vaccine programme managers: Use the serotype distribution to target the early vaccine rollout in the eight states where all four serotypes are present, as this is where the burden of severe co-infections is likely highest. Ensure that the chosen tetravalent vaccine demonstrates consistent efficacy against the dominant serotypes in each region, particularly DENV-2 and DENV-3. Strengthen the network of VRDLs to maintain real-time strain surveillance once vaccination begins.

For clinicians: In areas reporting all four serotypes, consider co-infection as a risk factor for severe dengue and incorporate it into patient management algorithms. Laboratory confirmation of serotypes can aid clinical decisions, and heightened vigilance for haemorrhagic manifestations and thrombocytopenia is warranted.

For the general population: Even with a vaccine on the horizon, mosquito control remains the first line of defence. Eliminate stagnant water, use repellents and protective clothing, and seek medical attention early if a high fever develops. The vaccine will complement—not replace—these preventive measures, and its protection against local strains will depend on the state-level strategy informed by the new strain map.