Influenza seasons were detected via active surveillance for influenza-like-illness (ILI), defined as a fever > 38 °C and cough or sore throat. Study health workers examined participants with ILI and collected

nose and throat swabs. Investigation was enhanced during the first wave of pandemic H1N1 transmission (September–December 2009) when all members of ILI case households were swabbed daily for up to 15 days. Blood samples were collected for serology at baseline in December 2007 ABT 263 and between each confirmed influenza season (Table 1). Combined nose and throat swabs were assessed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR), according to WHO/US CDC protocols (CDC reference no. I-007-05, Accessed November 30, 2009, at http://www.who.int/csr/resources/publications/swineflu/CDCRealtimeRTPCR_SwineH1Assay-2009_20090430.pdf).

Viruses were isolated from participants’ swabs and propagated in MDCK cells. The HA genes of seasonal H1N1 and H3N2 isolates were amplified and DNA sequencing performed using a 3100 genetic analyzer and BigDye Terminator Mix v3.0 (Applied Biosystems Inc.). Genome sequences representing vaccine strains and some with >93% identity to isolates sequenced in this study were click here downloaded from the NCBI Influenza Virus Resource (http://www.ncbi.nlm.nih.gov/genomes/FLU/FLU.html). Alignment of multiple sequences was performed by the ClustalW method.22 Phylogenetic trees were constructed using the maximum likelihood and neighbor-joining methods in the PHYLIP software package (version 3.66, University of Washington, Seattle, WA).23 Seasonal H3N2 and B isolates also underwent thorough antigenic characterization by the WHO Collaborating

Center for Reference and Research in Influenza in Melbourne, Australia. One H1N1 isolate from 2008 to 2 from 2009 were assessed in HI assay with seasonal Progesterone H1N1 reference sera provided in the 2010–2011 WHO Influenza Reagent Kit For Identification of Influenza Isolates (produced and distributed by: WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza, Centers for Disease Control and Prevention, Atlanta, Georgia 30333, U.S.A). Venous blood was collected into heparin vacutainers for the first two collection times and into serum vacutainers for the last two collection times. Plasma or sera was separated within 4 h and stored at −20 °C. Paired plasma/sera were tested in hemagglutination inhibition (HI) assay as previously described.21 Seasonal influenza H1N1 and H3N2 viruses isolated from participants’ swabs and propagated in MDCK cells were used for HI assay with serum pairs spanning season 1. The same H1N1 virus was used to assess season 2 plasma whereas the H3N2 virus used (TX265) was isolated from a patient presenting in Hanoi in the same season, and propagated in embryonated hen’s eggs.