Return of pandemic H1N1 influenza virus

Abstract

BackgroundInfluenza 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.MethodsSamples 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.ResultsWhile 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.ConclusionsHigh 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12879-014-0710-1) contains supplementary material, which is available to authorized users.

Seasonal Influenza A H1N1pdm09 Virus and Severe Outcomes: A Reason for Broader Vaccination in Non-Elderly, At-Risk People

“…Both a decremental pattern of severe diseases in those aged under 65 years lasting up to a decade after the pandemic [43] and a similar epidemiology to that of seasonal influenza A(H3N2) and A (H1N1) virus have been anticipated for influenza A H1N1pdm [44]. Our study and others on the 2010–2011 post-pandemic period [3, 19] and in the 2012–2013 seasons [13] found a higher proportion of hospitalized severe cases due to influenza A(H1N1) that were older than in pandemics, but younger than in seasonal influenza, and frequently with underlying medical conditions [3, 13]. For the moment, the age distribution and clinical spectrum of seasonal influenza A(H1N1) remains different from those of A(H3N3) 4 years after the 2009 pandemics.…”

“…However, cross-reactive immunity among adults over 60 years may have contributed to relatively low levels of infection with influenza A H1N1pdm09 in older adults [11]. By between 2010 and 2012, approximately 50% of the population had already developed immunity against the virus [12], and more severe cases in older and people with comorbidities were observed in the following [13]. In the 2009 pandemics, only 48% of hospitalized patients suffered from comorbidities, whereas in the 2010–2012 period, this figure had risen to 75% [14].…”

“…A higher proportion of hospitalized patients with severe disease and fatal cases with influenza A(H1N1) have been found in the first post-pandemic seasons, 2010–2011 [3,19] and 2012–13 [13]. However, it remains unclear whether influenza A(H1N1) is more virulent than other seasonal viruses, as reported in specific populations in pandemics, such as young people, the obese, and pregnant women [20–22].…”

A(H1N1)pdm09 influenza infection: vaccine inefficiency

“…To avoid immune recognition, both the HA and the NA proteins are subjected to extensive antigenic changes, complicating the design of a universal vaccine aiming to target all influenza strains [6]. Thus, the primary preventive strategy currently available against influenza virus infection is a vaccine that is designed and administered each year [7].…”

“…In Austria, 18.3% of the influenza-infected individuals were A(H1N1)pdm09-positive in winter 2014–2015 [13]. During the post-pandemic period in Israel, 10% of the patients hospitalized in 2010–2011 winter season and 5% of those hospitalized in the 2011–2012 winter season, were infected with the A(H1N1)pdm09 virus [6]. These numbers rose in winter 2012–2013, when 20% of the hospitalized patients were infected with the A(H1N1)pdm09 influenza virus, 64.8% of whom were pregnant women [6].…”