Why, for most of India, April-May is the best time to get that influenza shot

Multi-city research led by NIV shows patterns of virus seasonality that should inform vaccination times.

Updated: June 23, 2015 05:47 AM IST

4 min read

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As the monsoon advances further into the Indian mainland, the bulk of the country, barring the far north, is staring at the onset of influenza season. But the best time to get vaccinated against the flu virus is probably gone — for this year at least.

DISTINCT PATTERNS
A detailed five-year, 10-city analysis by scientists at the National Institute of Virology (NIV), Pune, has identified distinct patterns of circulation of strains of the flu virus in India.

Three discrete patterns have emerged — cities with temperate seasonality (such as Srinagar) show the most vigorous circulation of the virus during December-April for most years, with discrete peaks from January-March; cities across the Indo-Gangetic heartland and the west coast (such as Delhi, Dibrugarh, Lucknow, Kolkata, Pune and Alappuzha) show an influenza peak in July-October, with an additional winter peak in places like Delhi, Nagpur and Pune; and, finally, cities like Chennai and Vellore, which see peaks during November-December.

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A cumulative analysis of data showed more than 3 in 5 cases in most cities barring Srinagar, Chennai and Vellore occurred between June and November, NIV Deputy Director Dr M S Chadda told The Indian Express. In Srinagar, on the other hand, more than 86 per cent of flu cases occurred during December-May. In Chennai and Vellore, the virus circulated the most vigorously during November-December, when these cities see some wet weather.

VACCINATION TIMING
The complex pattern of influenza circulation poses challenges for influenza vaccination. The NIV analysis, using data from 2009 to 2013, showed that cities with temperate seasonality benefitted the most from vaccination in September-October, while cities with flu peaks in the July-September monsoon season, saw the most gains from vaccination in April-May.

Thus, Srinagar should consider vaccination in November-December, Chennai and Vellore in October-November, and most of the rest of the country in April-May, the scientists have advised.

RESEARCH METHOD
Influenza surveillance is an important tool to identify emerging/re-emerging strains, and define seasonality. Given the diverse topography and climatic conditions of India, the study focussed on 10 cities representing geographically distinct regions of the country. Patients with influenza-like symptoms in out-patient departments of dispensaries and hospitals, and hospitalized patients with severe acute respiratory infections were enrolled.

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Of the 44,127 ‘Influenza like Illness’ (ILI) and ‘Severe Acute Respiratory Illness’ (SARI) cases, 6,193 (14 per cent) tested positive for the influenza virus. Overall, 69 per cent of positive cases were of influenza type A; 31 per cent were of type B. A/H1N1 (swine flu) accounted for 50.3 per cent of type A and A/H3 for 48 per cent, with some circulation (1.7 per cent) of seasonal H1 before the 2009 pandemic.

Predominant strains varied over the years. A/H1N1 and influenza B viruses were the predominant strains in 2010, and A/H3 and influenza B co-circulated in 2011 — however, their proportion varied from city to city.

In 2012, influenza B was the dominant strain with some circulation of A/H1N1. In 2013, H3 emerged as the dominant strain with co-circulation of A/H1N1. Notably, the predominant circulating strains in Delhi during the monsoon (June-August) were usually the dominant strains in Srinagar the following winter.

NIV conducted the study along with the Sher-i-Kashmir Institute of Medical Sciences (SKIMS), Srinagar, All India Institute of Medical Sciences, New Delhi, and the National Institute for Cholera and Enteric Diseases (NICED), Kolkata, among other institutes. The results of the study, Dynamics of Influenza Seasonality at Sub-regional Levels in India and Implications for Vaccination Timing, were published in the peer-reviewed medical journal PLOS ONE last month.

Anuradha Mascarenhas

Anuradha Mascarenhas is a Senior Editor at The Indian Express, based in Pune. With a career spanning three decades, she is one of the most respected voices in Indian journalism regarding healthcare, science and environment and research developments. She also takes a keen interest in covering women's issues . Anuradha has won several awards including the Press Council of India's national award for excellence in journalism under the gender based reporting category in 2019 and the Laadli Media award (gender sensitivity -2024). A recipient of the Lokmat journalism award (gender category-2022), she was also shortlisted for the RedInk awards for excellence in journalism-2021. Her debut book At The Wheel Of Research, an exclusive biography of Dr Soumya Swaminathan the inaugural chief scientist of World Health Organisation was also nominated in the Popular Choice Category of JK Paper AUTHER awards. She has also secured competitive fellowships including the Laadli Media Fellowship (2022), the Survivors Against TB – New Research in TB Media Fellowship (2023) and is part of the prestigious 2025 India Cohort of the WomenLift Health Leadership Journey.” Professional Background Education: A gold medalist in Communication and Journalism from Savitribai Phule Pune University and a Master’s degree in Literature. Author: She authored the biography At The Wheel Of Research, which chronicles the life and work of Dr. Soumya Swaminathan, the former Chief Scientist at the WHO. Key Focus: She combines scientific accuracy with storytelling, translating complex medical research into compelling public and human-interest narratives. Recent Notable Articles (Late 2025) 1. Cancer & Specialized Medical Care "Tata Memorial finds way to kill drug-resistant cancer cells" (Nov 26, 2025): Reporting on a breakthrough for triple-negative breast cancer, one of the most aggressive forms of the disease. ``Discipline, diet and purpose; How a 97-year-old professor defies ageing'' (Nov 15, 2025) Report about Prof Gururaj Mutalik, the first Head of Department at Pune's B J Government Medical College who at 97 credits his longevity to healthy habits and a strong sense of purpose. 2. Environmental Health (The "Breathless Pune" Series) "Long-term exposure even to 'moderate' air leads to chronic heart, lung, kidney issues" (Nov 26, 2025): Part of an investigative series highlighting that even "safe" pollution levels are damaging to vital organs. "For every 10 µg/m3 increase in PM2.5 level, there was 6-8% jump in medicine sales" (Nov 23, 2025): Using commercial data to prove the direct link between air quality and respiratory illnesses in Pune. 3. Lifestyle & Wellness News "They didn't let cancer, diabetes and heart disease stop them from travelling" (Dec 22, 2025): A collaborative piece featuring survivors who share practical tips for traveling with chronic conditions. ``At 17, his BP shot up to 200/120 mmHG; Lancet study flags why child and teen hypertension doubled between 2000 and 2020'' (Nov 12,2025)--A report that focusses on 17-year-old-boy's hypertensive crisis and reflects the rising global trend of high blood pressure among children and adolescents. 4. Scientific Recognition & Infrastructure "For promoting sci-comm, gender diversity: IUCAA woman prof highlighted in Nature" (Nov 25, 2025): Covering the global recognition of Indian women scientists in gender studies and physics. ``Pune researchers find a spiral galaxy like the Milky Way from early universe'' (December 3, 2025)- A report on how Indian researchers discovered a massive galaxy that existed when the universe was just 1.5 billion years old , one of the earliest to have been observed so far. Signature Beat: Health, Science & Women in Leadership Anuradha is known for her COVID-19 reportage, where she was one of the first journalists to provide detailed insights into the Covishield and Covaxin trials. She has a dedicated interest in gender diversity in health and science, often profiling women researchers who are breaking the "leaky pipeline" in STEM fields. Her writing style is scrupulous, often featuring interviews with top-tier scientists and health experts from various institutions X (Twitter): @runaanu ... Read More

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