Dynamics of influenza seasonality at sub-regional levels in India and implications for vaccination timing.
BACKGROUND
Influenza surveillance is an important tool to identify emerging/reemerging strains, and defining seasonality. We describe the distinct patterns of circulating strains of the virus in different areas in India from 2009 to 2013.


METHODS
Patients in ten cities presenting with influenza like illness in out-patient departments of dispensaries/hospitals and hospitalized patients with severe acute respiratory infections were enrolled. Nasopharangeal swabs were tested for influenza viruses by real-time RT-PCR, and subtyping; antigenic and genetic analysis were carried out using standard assays.


RESULTS
Of the 44,127 ILI/SARI cases, 6,193 (14.0%) were positive for influenza virus. Peaks of influenza were observed during July-September coinciding with monsoon in cities Delhi and Lucknow (north), Pune (west), Allaphuza (southwest), Nagpur (central), Kolkata (east) and Dibrugarh (northeast), whereas Chennai and Vellore (southeast) revealed peaks in October-November, coinciding with the monsoon months in these cities. In Srinagar (Northern most city at 34°N latitude) influenza circulation peaked in January-March in winter months. The patterns of circulating strains varied over the years: whereas A/H1N1pdm09 and type B co-circulated in 2009 and 2010, H3N2 was the predominant circulating strain in 2011, followed by circulation of A/H1N1pdm09 and influenza B in 2012 and return of A/H3N2 in 2013. Antigenic analysis revealed that most circulating viruses were close to vaccine selected viral strains.


CONCLUSIONS
Our data shows that India, though physically located in northern hemisphere, has distinct seasonality that might be related to latitude and environmental factors. While cities with temperate seasonality will benefit from vaccination in September-October, cities with peaks in the monsoon season in July-September will benefit from vaccination in April-May. Continued surveillance is critical to understand regional differences in influenza seasonality at regional and sub-regional level, especially in countries with large latitude span.
National Institute of Virology, Pune, India.
Mandeep S
MS
Chadha
National Institute of Virology, Pune, India.
Varsha A
VA
Potdar
Centers for Disease Control and Prevention, Atlanta, USA.
Siddhartha
S
Saha
Sheri-Kashmir Institute of Medical Sciences, Srinagar, India.
Parvaiz A
PA
Koul
All India Institute of Medical Sciences, New Delhi, India.
Shobha
S
Broor
All India Institute of Medical Sciences, New Delhi, India.
Lalit
L
Dar
National Institute of Cholera and Enteric Diseases, Kolkata, India.
Mamta
M
Chawla-Sarkar
Regional Medical Research Centre, Dibrugarh, India.
Dipankar
D
Biswas
King Institute of Preventive Medicine & Research, Chennai, India.
Palani
P
Gunasekaran
Christian Medical College, Vellore, India.
Asha Mary
AM
Abraham
Indira Gandhi Medical College, Nagpur, India.
Sunanda
S
Shrikhande
King George Medical University (KGMU), Lucknow, India.
Amita
A
Jain
National Institute of Virology, Alappuzha, India.
Balakrishnan
B
Anukumar
Centers for Disease Control and Prevention, Atlanta, USA.
Renu B
RB
Lal
National Institute of Virology, Pune, India.
Akhilesh C
AC
Mishra
D002947:Cities; D005843:Geography; D006801:Humans; D007194:India / Q000453:epidemiology; D009980:Influenza A virus / Q000502:physiology; D009981:Influenza B virus / Q000502:physiology; D007251:Influenza, Human / Q000453:epidemiology*; D011159:Population Surveillance; D012621:Seasons; D013997:Time Factors; D014611:Vaccination / Q000706:statistics & numerical data*