Background Respiratory syncytial virus (RSV) is one of the major causes of respiratory infection and complications in younger children and elderly. Genetic diversity of RSV A and RSV B was for the first time investigated in influenza like illness (ILI) cases, reported in the scope of the Portuguese Influenza Surveillance Programme, between 2010-2015. Materials and Methods During 2010-2015, nasopharyngeal swabs sent to the National Influenza Reference Laboratory from sentinel and Raquel Guiomar, Paula Cristóvão, Patrícia Conde, Pedro Pechirra Molecular epidemiology of Respiratory Syncytial Virus between 2010-2015, in Portugal National Institute of Health Dr. Ricardo Jorge, Lisbon, Portugal National Influenza Reference Laboratory, Infectious Diseases Department RSVB/Portugal/su416/2014-15 RSVB/Portugal/RL18/2014-15 RSVB/Portugal/su358/2014-15 RSVB/Portugal/su28/2014-15 RSVB/Portugal/ms27/2014-15 RSVB/Portugal/su243/2014-15 RSVB/Portugal/ms120/2014-15 89 96 JX885730 ON1 JX885731 ON1 RSVA/Portugal/su4/2012-13 RSVA/Portugal/su152/2011-12 RSVA/Portugal/eva52/2012-13 JX912357 ON1 AB761609 ON1A RSVA/Portugal/su53/2012-13 JN257694 ON1A 65 67 RSV A RSV B Winter seasons: Reference Laboratory from sentinel and non-sentinel network were tested for RSV A and RSV B by real time multiplex RT-PCR. Nucleotide sequence of a fragment of the hypervariable C- terminal region of the G protein gene and the phylogenetic analysis was performed for an half of detected RSV. Results Over the study period were detected 114 (5.2%) RSV in 2187 tested NPS. Of these 67 (59%) were from subtype A and 47 (41%) from subtype B. Circulation of RSV preceded or was coincident with the influenza epidemic period. RSV A was predominant in each winter with exception for 2014/2015 winter when RSV B was predominantly detected. Of the RSV positive samples, 53 (46,5%) were successfully sequenced and genetically characterized: 24 (45%) RSV A and 29 (55%) RSV B. RSV A clustered in two RSVB/Portugal/su406/2014-15 RSVB/Portugal/su61/2011-12 RSVB/Portugal/eva56/2011-12 RSVB/Portugal/eva219/2014-15 RSVB/Portugal/su254/2014-15 RSVB/Portugal/su353/2013-14 RSVB/Portugal/eva252/2014-15 RSVB/Portugal/su334/2014-15 RSVB/Portugal/eva75/2014-15 RSVB/Portugal/ms56/2013-14 HM459876 BA9 RSVB/Portugal/su22/2012-13 RSVB/Portugal/su389/2010-11 DQ227395_BA9 HM459871_BA8 HM459866_BA7 HM459886_BA10 FJ490355 BA10 FJ490354 BA10 RSVB/Portugal/su23/2014-15 RSVB/Portugal/su80/2014-15 RSVB/Portugal/eva34/2014-15 RSVB/Portugal/su43/2014-15 RSVB/Portugal/ms106/2014-15 RSVB/Portugal/eva258/2014-15 RSVB/Portugal/eva95/2014-15 RSVB/Portugal/eva71/2012-13 RSVB/Portugal/su276/2012-13 RSVB/Portugal/ms70/2011-12 AB175820_BA5 AY751111_BA6 AY927401_BA GU550483_BA KC297490_BA DQ227381_BA3 DQ227389_BA2 87 52 61 65 88 76 53 68 70 59 RSVA/Portugal/su96+/2012-13 JX912355 ON1 RSVA/Portugal/su75/2011-12 RSVA/Portugal/su65/2012-13 RSVA/Portugal/eva178/2012-13 RSVA/Portugal/su215/2013-14 RSVA/Portugal/su139/2013-14 RSVA/Portugal/su337/2013-14 RSVA/Portugal/Eva233/2014-15 RSVA/Portugal/su202/2013-14 RSVA/Portugal/ms45/2013-14 RSVA/Portugal/su213/2013-14 RSVA/Portugal/su47/2013-14 KF246641 ON1C KF246638 ON1C RSVA/Portugal/Eva248/2014-15 RSVA/Portugal/su277/2013-14 RSVA/Portugal/su93+/2012-13 RSVA/Portugal/Su414/2014-15 RSVA/Portugal/su446/2010-11 KC297377 NA1 KC297305 NA1 RSVA/Portugal/su46+/2011-12 RSVA/Portugal/eva251/2010-11 BJ/29240 NA3 KC297248 NA3 KC297251 NA1 KC297301 NA1 BJ/22998 NA1 NG01604 NA1 RSVA/Portugal/su494/2010-11 AB603443 NA2 AY524595 GA2 AY114149 GA2 BJ/F5166 NA4 BJ/36578 NA4 KC297324 NA4 AF512538 GA2 AF233900 GA2 AF065258 GA2 Z33425 GA2 Z33422 GA2 Z33415 GA2 CN2395 GA3 AF233905 GA3 100 89 99 100 57 56 78 50 87 92 70 52 88 75 64 2010/2011 2011/2012 2012/2013 2013/2014 2014/2015 ON1 NA1 BA9 BA10 For additional information, please contact Raquel Guiomar (National Institute of Health Dr. Ricardo Jorge / Portuguese NIC) raquel.guiomar@insa.min-saude.pt ; Tel.: (+351) 217519216 genotypes (Figure 1): most viruses (n=20; 83%) belonged to ON1 genotype while 4 (17%) viruses belonged to NA1 genotype. Since 2012/2013 season, only ON1 genotype was detected. All RSV B present a BA-like genotype (Figure 2). Most of them (19/29; 66%) clustered within BA9 genotype, the other strains clustered within BA10 genotype. BA9 and BA10 genotypes were detected over all the study period. Conclusions Our study highlights the importance of RSV in ILI cases, showing a seasonal circulation each winter season during influenza epidemic. RSV accounted for 5.2% of the cases reported in the scope of influenza surveillance, assuming a huge importance in young children and elderly. Molecular data for RSV A revealed co circulation of NA1 and ON1 between 2010-2012. After this period, ON1 was exclusively detected suggesting a strain replacement by this antigenically advantageous genotype. Globally ON1 is also predominantly detected. For RSVB subtype was observed the circulation of only BA genotypes (BA9 and BA10), which were first identified in 1999 in Buenos Aires and since then are predominant in many countries. Acknowledgments The authors would like to thank to the GP Sentinel network, Non-sentinel Influenza Network (Emergency and Obstetric Units) that contributed to the clinical and virological components of the National Influenza Surveillance Programme during 2010-2015 seasons. Figure 1 – Neighbour Joining phylogenetic tree based on the C-terminal region of G protein coding sequence of detected RSV A. Was used the Kimura-2 parameter substitution model, bootstrap values above 50 are shown (500 replicates). DQ227389_BA2 DQ227408_BA4 AY333364_BA1 AF348821_SAB2 AF348813_SAB3 JN119979_SAB4 AY660682_SAB1 KC297462_URU2 AF065251CH93_GB2 AF065250_GB1 M73542_GB1 M73540_GB1 AY488803_URU2 M17213 96 95 69 57 58 54 0,02 AF233905 GA3 AF348804 GA7 AF233904 GA7 Z33417 GA7 Z33411 GA7 AY524593 GA5 AY114150 GA5 Z33493 GA5 AF233916 GA5 X73352 GA5 AF065254 GA4 AF348807 SAA1 AF348810 SAA1 AF233918 GA6 X73350 GA1 AF233917 GA1 Z33431 GA1 X73354 GA1 M11486 60 100 93 59 85 99 98 100 95 89 71 100 0.02 Figure 2 – Neighbour Joining phylogenetic tree based on the C-terminal region of G protein coding sequence of detected RSV B. Was used the Kimura-2 parameter substitution model, bootstrap values above 50 are shown (500 replicates).