Swine influenza

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Swine influenza is an infection of a host animal by any one of several specific types of swine influenza virus. In 2009 the media labeled as "swine flu" a disease caused by 2009's new strain of swine-origin A/H1N1 pandemic virus, which is transmitted between humans.


  • I would tell members of my family -- and I have -- I wouldn't go anywhere in confined places now....It's not (just) going to Mexico, it's you're in a confined aircraft when one person sneezes it goes all the way through the aircraft.

Jessica Y. Wong, Heath Kelly, Dennis K. M. Ip, Joseph T. Wu, Gabriel M. Leung, and Benjamin J. Cowling; “Case fatality risk of influenza A (H1N1pdm09): a systematic review.”, Epidemiology 24, 830–841 (2013).[edit]

  • In April 2009 the World Health Organization declared a formal “public health emergency of international concern,” marking the start of an international public health response to the first influenza pandemic of the 21st Century. One of the immediate priorities was to quantify the transmissibility of the new pandemic influenza A (H1N1pdm09) virus (denoted H1N1pdm09 hereafter) and the seriousness of infection with this virus, because these two epidemiologic measures in combination determine the severity of the pandemic in the absence of control measures. Whereas a number of transmissibility estimates, based on the reproduction number R, were published with broad agreement from the early stages of the pandemic, there was far greater difficulty in estimating the seriousness of infections. In the report of the World Health Organization’s Review Committee on the functioning of the 2005 International Health Regulations in relation to H1N1pdm09, Fineberg et al. identified “the absence of a consistent, measurable and understandable depiction of severity of the pandemic” as one of the major shortcomings of the international public health response.
  • There is very substantial heterogeneity in published estimates of case fatality risk for H1N1pdm09, ranging from <1 to >10,000 per 100,000 infections. Large differences were associated with the choice of case definition (denominator). Because influenza virus infections are typically mild and self-limiting, and a substantial proportion of infections are subclinical and do not require medical attention, it is challenging to enumerate all symptomatic cases or infections. In 2009, some of the earliest available information on fatality risk was provided by estimates based primarily on confirmed cases. However, because most H1N1pdm09 infections were not laboratory-confirmed, the estimates based on confirmed cases were up to 500 times higher than those based on symptomatic cases or infections. The consequent uncertainty about the case fatality risk — and hence about the severity of H1N1pdm09 — was problematic for risk assessment and risk communication during the period when many decisions about control and mitigation measures were being made.
  • Arguably one of the ideal estimates of seriousness of infection is the infection fatality risk, i.e. the case fatality risk based on infections as denominator. This risk estimate can be compared across populations without concerns over differences in symptom perceptions and reporting, health-care seeking behaviors, or laboratory-testing capacity. Estimates of the infection fatality risk can also be used in combination with estimates of transmissibility to directly inform predictions of population impact. The use of the term “infection fatality” differentiates this risk estimate from the case fatality risk since the asymptomatic, undetected and undiagnosed infections included in the denominator would not appear as “cases” under typical case definitions. However, few estimates of the infection fatality risk were available for H1N1pdm09, and none was available early in the pandemic. Serologic studies, or estimates of the cumulative incidence of symptomatic infections in a population adjusted for the proportion of infections that are asymptomatic, can be used to estimate the denominator for the infection fatality risk. However, one has to define how to estimate infection rates from serologic data, and there is not yet consensus on the best approach.
  • In addition to differences in case fatality risk estimates due to the differences in case definition (denominator), the definition of the numerator is also an important issue. Almost all of the studies in our review based the numerator on deaths among patients with laboratory-confirmed influenza infection. In contrast, most estimates of the population impact of seasonal influenza epidemics have been based on estimation of the number of excess deaths associated with influenza (i.e. estimated deaths), with the greatest annual impact in the elderly — despite influenza virus infections rarely being confirmed in this age group. The use of excess deaths rather than laboratory-confirmed deaths in the numerator of the infection fatality risk would theoretically be justified because the denominator includes all infections and not only those with a positive laboratory result. For a similar reason, deaths of patients with laboratory-confirmed infection might be a more appropriate numerator for the case fatality risk based on symptomatic case denominators.
  • In preparation for the next influenza pandemic, it is essential to reach a consensus on how to define and measure the seriousness of infection (an important indicator of the severity of the pandemic), and whether the analysis can be based entirely on estimates of age-specific risk of death among cases. The consistent estimates of the infection fatality risk at around 1 to 10 deaths per 100,000 infections identified in our review may represent the seriousness of H1N1pdm09 in developed countries where data were available. Similar estimates for seasonal influenza viruses, however, are not available for comparison, and neither are estimates from less developed countries in which the seriousness profile would likely be higher.

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