title: Return of pandemic H1N1 influenza virus.
abstract: BACKGROUND
  Influenza pandemics are usually caused by the re-assortment of several influenza viruses, results in the emergence of new influenza virus strains that can infect the entire population. These pandemic strains, as well as seasonal influenza viruses, are subjected to extensive antigenic change that has, so far, prevented the generation of a universal vaccine.
  
  
  METHODS
  Samples of patients hospitalized due to infection with the pandemic H1N1 influenza virus (A(H1N1)pdm09) from 2009, when the virus first appeared, until 2013 were analyzed.
  
  
  RESULTS
  While many patients were hospitalized in 2009 due to infection with the pandemic H1N1 influenza virus, only small percentages of patients were hospitalized later in 2010-2012. Surprisingly, however in 2012-2013, we noticed that the percentages of patients hospitalized due to the pandemic H1N1 influenza infection increased significantly. Moreover, the ages of hospitalized patients differed throughout this entire period (2009-2013) and pregnant women were especially vulnerable to the infection.
  
  
  CONCLUSIONS
  High percentages of patients (especially pregnant women) were hospitalized in 2013 due to the A(H1N1)pdm09 infection, which may have been enabled by an antigenic drift from those which circulated at the onset of the pandemic.
authors.affiliation: Central Virology Laboratory, Ministry of Health, Public Health Services, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. hilda.sherbany@sheba.health.gov.il.
authors.forename: Hilda
authors.initials: H
authors.lastname: Sherbany
authors.affiliation: WHO Collaborating Centre for Reference and Research on Influenza, Division of Virology, MRC National Institute for Medical Research, Mill Hill, London, NW7 1AA, UK. jmccaul@nimr.mrc.ac.uk.
authors.forename: John
authors.initials: J
authors.lastname: McCauley
authors.affiliation: Central Virology Laboratory, Ministry of Health, Public Health Services, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. talsegman@gmail.com.|The Mina & Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel. talsegman@gmail.com.
authors.forename: Tal
authors.initials: T
authors.lastname: Meningher
authors.affiliation: The Mina & Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel. musa.hindiyeh@sheba.health.gov.il.
authors.forename: Musa
authors.initials: M
authors.lastname: Hindiyeh
authors.affiliation: Israel Center for Disease Control, Ministry of Health, Gertner Institute for Health Policy Research, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. Rita.Dichtiar@MOH.HEALTH.GOV.IL.
authors.forename: Rita
authors.initials: R
authors.lastname: Dichtiar
authors.affiliation: Israel Center for Disease Control, Ministry of Health, Gertner Institute for Health Policy Research, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. Michal.Perry@MOH.HEALTH.GOV.IL.
authors.forename: Michal Perry
authors.initials: MP
authors.lastname: Markovich
authors.affiliation: Central Virology Laboratory, Ministry of Health, Public Health Services, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. ella.mendelson@sheba.health.gov.il.|Department of Epidemiology and Preventive Medicine, School of Public Health, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel. ella.mendelson@sheba.health.gov.il.
authors.forename: Ella
authors.initials: E
authors.lastname: Mendelson
authors.affiliation: Central Virology Laboratory, Ministry of Health, Public Health Services, Chaim Sheba Medical Center, Tel Hashomer, Ramat-Gan, Israel. michal.mandelboim@sheba.health.gov.il.
authors.forename: Michal
authors.initials: M
authors.lastname: Mandelboim
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D021821:Communicable Diseases, Emerging / Q000453:epidemiology / Q000517:prevention & control; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data / Q000639:trends; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics / Q000276:immunology / Q000302:isolation & purification*; D007252:Influenza Vaccines / Q000627:therapeutic use; D007251:Influenza, Human / Q000453:epidemiology* / Q000517:prevention & control; D007557:Israel / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D011247:Pregnancy; D012189:Retrospective Studies; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines