title: Frequency of influenza H3N2 intra-subtype reassortment: attributes and implications of reassortant spread.
abstract: Increasing evidence suggests that influenza reassortment not only contributes to the emergence of new human pandemics but also plays an important role in seasonal influenza epidemics, disease severity, evolution, and vaccine efficacy. We studied this process within 2091 H3N2 full genomes utilizing a combination of the latest reassortment detection tools and more conventional phylogenetic analyses.
  
  
  
  We found that the amount of H3N2 intra-subtype reassortment depended on the number of sampled genomes, occurred with a steady frequency of 3.35%, and was not affected by the geographical origins, evolutionary patterns, or previous reassortment history of the virus. We identified both single reassortant genomes and reassortant clades, each clade representing one reassortment event followed by successful spread of the reassorted variant in the human population. It was this spread that was mainly responsible for the observed high presence of H3N2 intra-subtype reassortant genomes. The successfully spread variants were generally sampled within one year of their formation, highlighting the risk of their rapid spread but also presenting an opportunity for their rapid detection. Simultaneous spread of several different reassortant lineages was observed, and despite their limited average lifetime, second and third generation reassortment was detected, as well as reassortment between viruses belonging to different vaccine-associated clades, likely displaying differing antigenic properties. Some of the spreading reassortants remained confined to certain geographical regions, while others, sharing common properties in amino acid positions of the HA, NA, and PB2 segments, were found throughout the world.
  
  
  
  Detailed surveillance of seasonal influenza reassortment patterns and variant properties may provide unique information needed for prediction of spread and construction of future influenza vaccines.
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA. irina.maljkovicberry.ctr@mail.mil.
authors.forename: Irina
authors.initials: I
authors.lastname: Maljkovic Berry
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Melanie C
authors.initials: MC
authors.lastname: Melendrez
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Tao
authors.initials: T
authors.lastname: Li
authors.affiliation: Operational Infectious Diseases Directorate, Naval Health Research Center, San Diego, CA, USA.
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Hawksworth
authors.affiliation: Operational Infectious Diseases Directorate, Naval Health Research Center, San Diego, CA, USA.
authors.forename: Gary T
authors.initials: GT
authors.lastname: Brice
authors.affiliation: Operational Infectious Diseases Directorate, Naval Health Research Center, San Diego, CA, USA.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Blair
authors.affiliation: US Naval Medical Research Unit -6, Lima, Peru.
authors.forename: Eric S
authors.initials: ES
authors.lastname: Halsey
authors.affiliation: US Naval Medical Research Unit -6, Lima, Peru.
authors.forename: Maya
authors.initials: M
authors.lastname: Williams
authors.affiliation: Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Stefan
authors.initials: S
authors.lastname: Fernandez
authors.affiliation: Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.|Present Address: International Vaccine Institute, Seoul, Republic of Korea.
authors.forename: In-Kyu
authors.initials: IK
authors.lastname: Yoon
authors.affiliation: Office of Medical Services, US Department of State, Washington, DC, USA.
authors.forename: Leslie D
authors.initials: LD
authors.lastname: Edwards
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Robert
authors.initials: R
authors.lastname: Kuschner
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Xiaoxu
authors.initials: X
authors.lastname: Lin
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Thomas
authors.affiliation: Walter Reed Army Institute of Research, Silver Spring, MD, USA.
authors.forename: Richard G
authors.initials: RG
authors.lastname: Jarman
mesh_terms: D019143:Evolution, Molecular; D016679:Genome, Viral / Q000235:genetics; D006801:Humans; D053122:Influenza A Virus, H3N2 Subtype / Q000145:classification / Q000235:genetics*; D007251:Influenza, Human / Q000635:transmission / Q000821:virology; D010802:Phylogeny
keywords: H3N2; Influenza; Reassortment; Spread