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The G4 virus, also known as the "G4 swine flu virus" (G4) and "G4 EA H1N1", is a swine influenza virus strain discovered in China. [68] The virus is a variant genotype 4 (G4) Eurasian avian-like (EA) H1N1 virus that mainly affects pigs, but there is some evidence of it infecting people. [ 68 ]
The virus is a novel strain of the influenza virus, [2] for which existing vaccines against seasonal flu provided no protection. A study at the U.S. Centers for Disease Control and Prevention (CDC) published in May 2009 found that children had no preexisting immunity to the new strain but that adults, particularly those over 60, had some degree of immunity.
The H1N1 form of swine flu is one of the descendants of the strain that caused the 1918 flu pandemic. [ 71 ] [ 72 ] As well as persisting in pigs, the descendants of the 1918 virus have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza. [ 72 ]
The 2009 swine flu pandemic, caused by the H1N1/swine flu/influenza virus and declared by the World Health Organization (WHO) from June 2009 to August 2010, was the third recent flu pandemic involving the H1N1 virus (the first being the 1918–1920 Spanish flu pandemic and the second being the 1977 Russian flu).
Influenza A virus structure. The influenzavirus virion is pleomorphic; the viral envelope can occur in spherical and filamentous forms. In general, the virus's morphology is ellipsoidal with particles 100–120 nm in diameter, or filamentous with particles 80–100 nm in diameter and up to 20 μm long. [5]
The Centers for Disease Control and Prevention (CDC) identified the first two A/09(H1N1) swine flu cases in California on April 17, 2009, via the Border Infectious Disease Program, [135] for a San Diego County child, and a naval research facility studying a special diagnostic test, where influenza sample from the child from Imperial County was tested. [136]
Community outbreaks, June 2009 Confirmed cases by state, June 3, 2009. This article covers the chronology of the 2009 novel influenza A pandemic.Flag icons denote the first announcements of confirmed cases by the respective nation-states, their first deaths (and other major events such as their first intergenerational cases, cases of zoonosis, and the start of national vaccination campaigns ...
In the 21st century, a strain of H1N1 flu (since titled H1N1pdm09) which was antigenically very different from previous H1N1 strains, leading to a pandemic in 2009. Because of its close resemblance to some strains circulating in pigs, this became known as "Swine flu" [46] Influenza A virus continues to circulate and evolve in birds and pigs.