Multisite virological influenza surveillance in India: 2004-2008

Influenza surveillance is important to identify circulating, emerging/reemerging strains and unusual epidemiological trends. With these objectives, a multisite human influenza surveillance network was initiated in India in 2004.

Epidemiologic data and throat swabs for laboratory testing were collected from patients with influenza-like illness (ILI) and severe acute respiratory infections (SARI). Virus isolation was carried out in Madin-Darby canine kidney cells and strains identified by hemagglutination inhibition assay. Meteorological data were collected.

From September 2004 to December 2008, 617 (4·43%) of 13928 cases yielded isolates: 27·8% were influenza A(H1N1), 29·8% were type A(H3N2), and 42·3% were type B. The yearly type and subtype distribution varied significantly from site to site. Peak influenza activity was observed from June to August in Delhi, Pune, and Kolkata and October to December in Chennai. Maximum influenza activity was seen during the rains in Delhi, Pune, Chennai, and Kolkata in correlation with virus isolations. Multivariate analysis of ILI cases showed chill/rigors, cough, fatigue, and ILI in family, correlated positively with isolation. Genetic analysis of Indian isolates revealed that viruses matched with vaccine strains by and large. Overlapping between circulating and vaccine component strains of consecutive years was also observed.

Seasonal influenza A(H1N1), H3N2, and type B co-circulated in all regions without any particular pattern of movement of any subtype. Year-round limited influenza activity with peaks during rains was observed. Genetic drifts and varying seasonality in different parts of the country suggest that a staggered timing of vaccination may be appropriate for India.