Laboratory surveillance data from the national VRDLs network indicated that the contribution of influenza to ILI was 25% which correlates with several studies like Maman et al. (7) which showed 25%, Bellei et al, (8) Nandhini et al.  (9) and  Chadha et al.(10) which showed 21% of influenza cases.

According to WHO update 2011 (11) during post-pandemic period influenza A/pdmH1N1 is expected to circulate as a seasonal virus for some years to come. Since the initial pandemic outbreak, the 2009pdmH1N1 has replaced the prior seasonal H1N1 and established itself in the human population. This is largely due to sequence evolution of the hemagglutinin (HA) protein, whose activity critically governs the receptor binding, fusion, and transmission properties of the virus (12). Influenza A H1N1 is predominant type of influenza in our study; this observation was similar to that of other studies in India like Choudhry A et al (13), Dangi T et al (14) and Nandhini et al. (9)

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During 2014-15 influenza activity in the northern hemisphere has been high, predominantly due to influenza A(H3N2). However, some countries in Northern Africa, Middle East, and notably India reported an increase in influenza A(H1N1)pdm09 activity (15). VRDLN surveillance data suggest there was sharp rise in the number of Influenza A H1N1 cases in 2015, same finding was seen in IDSP report (17). During early 2015, a sudden increase in pandemic influenza activity was observed in almost all states of India and its surroundings. There have been reports of severe second pandemic wave in Asia and Europe during earlier pandemics too (16). The pandemic virus A(H1N1)pdm09 also showed more virulence in 2015 compared to 2009–2010 in some of the Asian countries including India (17) (18). The death toll in India during 2015 crossed the number of deaths occurred during the pandemic of 2009–2010 suggesting increased virulence of 2015 A(H1N1)pdm09virus (19). The pandemic A(H1N1)pdm09 Influenza A virus has shown high genetic diversity since its emergence in 2009 (20). Though the 2015 Indian-origin strains appear to have not undergone any reassortment, the probability of emergence of a more virulent pandemic A(H1N1)pdm09 virus due to continuous antigenic drift cannot be ruled out (21). Moreover high population density in India facilitates the person-to-person transmission, and create sample opportunities for these mutated variants to sustain and become dominant (21). India reported very little activity in 2016, in sharp contrast with 2015 when there was a peak of influenza activity (over 30% positivity) in February and March of 2015. (22)

The presentation of seasonal influenza ranges from an asymptomatic infection to a fulminant illness, depending on the characteristics of both the host and virus.(2),(23),(24),(25) Symptoms appear suddenly after an incubation period of 1–2 days and are characterised by various systemic features, including fever, chills, headache, myalgia, malaise, and anorexia, accompanied by respiratory symptoms, including non-productive cough, nasal discharge, and sore throat (2, 23),(24),(25). In current study, most common clinical presentation was fever (77% cases) similar to other studies from India (13, 26, 27) followed by cough in 75% cases. There was no significant difference in the proportion of females compared to males enrolled in this study. No gender specificity of infection was also seen in study by Mukherjee et al.(21)

1 It is proposed that children and the elderly are prone to seasonal influenza infection (28), but in our study positivity for H1N1 was seen highest in age group 30-60 yrs, ranging from 26-29% similar finding was seen in the study by Nandhini et al (9) 1 which showed 52% of patients were in the age group of 20–49 years . Although Influenza A H3N2 and influenza B were seen highest in children and adults.

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