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.
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.
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.
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.
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.