2009 H1N1 influenza: a twenty-first century pandemic with roots in the early twentieth century

Abstract

A swine-origin H1N1 triple-reassortant influenza A virus found to be a distant relative of the 1918 "Spanish flu" virus emerged in April 2009 to give rise to the first influenza pandemic of the 21st century. Although disease was generally mild and similar to seasonal influenza, severe manifestations including respiratory failure were noted in some, particularly those with underlying conditions such as asthma, pregnancy and immunosuppression. Children and younger adults accounted for most cases, hospitalizations and deaths. A reverse transcriptase-polymerase chain reaction assay was superior to antigen-based rapid tests for diagnosis. All 2009 H1N1 pandemic influenza strains were susceptible to 1 or more neuraminidase inhibitors. Monovalent, unadjuvanted 2009 H1N1 vaccines were licensed in the United States in September 2009 and initially targeted to younger individuals, pregnant women, caretakers of infants and healthcare providers. The 2009 H1N1 pandemic highlights the need for modernization of influenza vaccines, improved diagnostics and more rigorous evaluation of mitigation strategies.

Figure 1

2009 H1N1 influenza A virus. The 8 gene segments in descending order according to size are shown in the center of the virus. Gene segments with vertical hatched lines are of avian origin; the white bar indicates human origin; black bars indicate North American classical swine origin; and solid gray bars indicate gene segments of Eurasian swine origin. Hemagglutinin (HA, in black) and neuraminidase (NA, in gray) surface proteins surround the viral particle. Gene segments encode for the following proteins: polymerase PB2, polymerase PB1, polymerase PA, hemagglutinin (HA), nuclear protein (NP), neuraminidase (NA), matrix proteins (M), and nonstructural proteins (NS)., ,

Figure 2

Prevalence of pandemic 2009 H1N1 by age group. Estimates of the prevalence of pandemic 2009 H1N1 in the United States, April 2009 to July 2009, by age group (with permission, based upon median case estimates in Reed et al). Estimated 2009 H1N1 cases are shown in the closed circles and the estimated hospitalizations are shown in the closed squares.