Investigators from the Centers for Disease Control and Prevention and 25 other public health agencies around the world sought to determine the genetic origin and the antigenic characteristics of the 2009 A(H1N1) influenza virus, commonly referred to as “swine flu.” Specifically, they used a genetic-similarity study to determine which influenza strains previously identified had combined to produce the new A(H1N1) flu virus.
Genetic sequences of 17 flu strains isolated from Mexico and 59 strains from 12 states in the United States were compared with previously isolated strains of human seasonal influenza, influenza circulating in swine populations, and influenza circulating in avian populations. Additionally, experimental assays were used to determine which vaccine preparation would be most effective for immunization to the 2009 A(H1N1) flu strain.
The genetic-similarity study found that the 2009 A(H1N1) flu was derived from the combination of 4 separate strains of influenza, each circulating in swine populations, including an A(H1N1) strain that has been circulating in swine populations since around the time of the Spanish flu outbreak of 1918. The researchers determined that because of the length of time that classic swine A(H1N1) has been circulating in swine populations vs human seasonal A(H1N1), the antigenic profile of swine A(H1N1) has drifted sufficiently so that vaccines prepared for human A(H1N1) would have little or no efficacy.
Viewpoint
Although the mortality rate of the 2009 A(H1N1) flu is much lower than originally feared, the circulation of a previously unrecognized novel H1N1 strain in humans highlights the shortcomings of current influenza surveillance efforts worldwide. Indeed, the antigenic difference between gene segments circulating in swine populations vs humans suggests the possibility that swine populations may be a reservoir for influenza strains with pandemic potential.
Garten RJ, Davis CT, Russell CA, et al Antigenic and genetic characteristics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science. 2009 Jul 10;325(5937):197-201. PubMed PMID: 19465683. View Abstract
