Why H3N2 Flu Cases Are Rising Even Before Peak Summer: Doctors Share Why, Preventive Tips

This year's influenza season is behaving differently. Across regions from the northern hemisphere to India, H3N2, a subtype of influenza A, is emerging earlier and more actively than usual. Traditionally, flu seasons follow predictable, cooler weather patterns with peaks in late autumn and winter, but variations in climate, viral evolution, and human behaviour are reshaping this trend. Public health bodies, including the World Health Organization (WHO), report that seasonal influenza activity, particularly A(H3N2) viruses, has been climbing in parts of the world even before typical seasonal peaks. 

In regions like Delhi-NCR, a significant proportion of households are reporting flu-like symptoms, with H3N2 identified as a dominant cause of these infections. Medical experts such as Dr. A. K. Shukla, Sr. Consultant Physician, Internal Medicine/Physicians, Kailash Hospital, Noida and Dr. Sandeep Bhatnagar, Senior Director and HOD - Internal Medicine, Paras Health Udaipur, attribute this early surge to erratic seasonal shifts, immune system stress, social behaviour changes, and environmental factors. Their clinical observations, combined with global surveillance data, paint a picture of a flu season that's both early and unusually dynamic, challenging conventional assumptions about influenza timing and transmission. 

Early Onset of H3N2 Influenza Virus: What Surveillance Data Shows

Seasonal influenza activity has historically followed a seasonal pattern, increasing in late fall and winter, and declining by spring. However, surveillance data from the WHO's Global Influenza Surveillance and Response System (GISRS) indicates early influenza activity and rising A (H3N2) detection in several regions well outside peak months.

In many northern hemisphere countries, global influenza activity began increasing in the latter half of 2025, with H3N2 viruses predominating over other subtypes. Experts have also noted the emergence of subclade variants (like "subclade K") that can circulate earlier and spread efficiently across populations.

Regionally, areas like Delhi-NCR saw a rise in households reporting viral symptoms, with doctors linking this surge to H3N2. Surveys showed up to 69% of households reporting illnesses consistent with flu symptoms, a significant jump from earlier counts.

Also Read: Flu Surge In Delhi: Doctors Warn Of High Fever, Persistent Cough, And Relapses; All Symptoms Listed

Why Seasonal Shifts Matter: Weather, Immunity and Viral Spread

From a meteorological perspective, influenza virus transmission is influenced by temperature, humidity, and atmospheric pressure. Climate analyses show correlations between these environmental variables and influenza peaks. For instance, colder nights followed by warm days, conditions common during transitional seasons, may facilitate viral survival and transmission in the airways.

Dr. A. K. Shukla points out that erratic weather destabilizes people's internal adaptation mechanisms, temporarily weakening immune defenses and making populations more vulnerable to viruses like H3N2. Fluctuating humidity and wind patterns can irritate airways, creating favourable hosts for viral infection. Similarly, exposure to pollution and dust during seasonal shifts exacerbates respiratory irritation and susceptibility.

Meanwhile, Dr. Sandeep Bhatnagar highlights that the interplay between air conditioning use during warm hours and cooler outdoor temperatures stresses the respiratory system, hampering mucosal immunity, our first line of defense against airborne viruses. This interplay is more common in transitional periods, allowing flu viruses to spread before the classic winter peak.

Human Behaviour and Lifestyle Factors Amplify Risk

Seasonal transitions don't just affect the virus, they also impact human behaviour. During fluctuating weather:

• Social interactions increase (travel, gatherings, live events) as people adapt to changing conditions, heightening transmission.
• Indoor crowding rises, especially where ventilation is poor.
• Sleep, hydration, and activity levels suffer, weakening immune resilience.
• Post-viral vulnerability from recent illnesses compounds risk, as recovering individuals may have transiently reduced immunity.

These lifestyle factors create ideal conditions not just for circulation, but for persistence and spread of influenza viruses during months when they might otherwise be dormant.

Clinical Impact and Symptoms to Watch

The H3N2 subtype is known to cause significant influenza activity, especially among children, the elderly, and those with chronic health conditions. Symptoms often include high fever, persistent cough, sore throat, body aches, and fatigue, and can last longer than typical seasonal flu.

While this strain has not shown higher inherent severity than other influenza viruses, its early and widespread circulation increases the cumulative impact on health systems, especially if large swaths of the population become infected simultaneously.

Also Read: Is India Prepared For Influenza? Central Disease Control Body, WHO Assess

Prevention: What Experts Recommend

Doctors emphasize that basic preventive measures still matter most:

• Annual influenza vaccination: Even if vaccine match varies, it significantly reduces risk of severe disease and hospitalization.
• Good respiratory hygiene: Covering coughs and sneezes, and mask use in crowded areas.
• Hand hygiene and hydration: These support immune defenses.
• Seek early medical advice if high fever, persistent symptoms, or breathing difficulties occur.

These steps are crucial for high-risk groups, including older adults and those with pre-existing conditions.

The early rise in H3N2 flu cases reflects a convergence of seasonal shifts, climatic variability, viral evolution, and human behavioural factors. With global surveillance confirming increased influenza activity outside typical peak months, healthcare experts are urging vigilance. Understanding these drivers, from erratic temperature changes to immune modulation, empowers individuals to act proactively and protect themselves as seasonal patterns evolve.

Disclaimer: This content including advice provides generic information only. It is in no way a substitute for a qualified medical opinion. Always consult a specialist or your own doctor for more information. NDTV does not claim responsibility for this information.