A few weeks ago I wrote about a paper detailing the evolutionary origins of swine flu. Now a second paper has been published on this topic (this one out this week in Science). This study comes from a different group of researchers (a long list of mostly US-based researchers, led by Rebecca Garten and Todd Davis of the Center for Disease Control of Prevention in Atlanta, GA), and differs slightly from the previous study in that the researchers have sequenced more isolates of the pandemic strain, which they refer to as 2009 A(H1N1), so have been able to look in more depth at genetic variation within the strain. They also report on the antigenic characteristics of the virus – that is, how it reacts against antibodies.
Garten and colleagues sequenced a total of 76 isolates of 2009 A(H1N1) – 17 from Mexico and 59 from the US, and compared them to other influenza viruses. Not surprisingly, they came to the same conclusions as the previous study regarding the evolutionary origin of the component parts of the virus – that it is a triple reassortment virus, and that its closest relatives have been circulating undetected in swine populations for about 10 years. As with previous study, there was no evidence of mutations that are likely to result in increased virulence of the virus, or molecular markers that are associated with adaptation to human host.
The isolates they sequenced were 99.9% identical, with only 5 minor genetic variants among them. This low level of genetic variation within humans suggests that its introduction into humans probably involved either a single transmission event, or multiple transmissions of very similar viruses. As viruses evolve extremely quickly the small amount of genetic variation they did find may have arisen since its transmission to humans.
The antigenic properties of 2009 A(H1N1) were tested by raising antibodies against the HA protein of this strain in ferrets, and testing how well these antibodies react against other isolates of the pandemic strain. The HA protein is the critical one for flu virulence and the target of flu vaccines, as humans have little or no immunity to this protein. Garten and colleagues found that antigenically, the pandemic viruses are homogeneous, in fact they exhibit less variation than seen with typical seasonal influenza. This is good news for vaccine production, as one vaccine will be able to target all the current variation existing within the virus. However, antibodies against seasonal flu didn’t react with swine flu, indicating that being vaccinated against seasonal flu is unlikely to help you resist swine flu.
Garten, R., Davis, C., Russell, C., Shu, B., Lindstrom, S., Balish, A., Sessions, W., Xu, X., Skepner, E., Deyde, V., Okomo-Adhiambo, M., Gubareva, L., Barnes, J., Smith, C., Emery, S., Hillman, M., Rivailler, P., Smagala, J., de Graaf, M., Burke, D., Fouchier, R., Pappas, C., Alpuche-Aranda, C., Lopez-Gatell, H., Olivera, H., Lopez, I., Myers, C., Faix, D., Blair, P., Yu, C., Keene, K., Dotson, P., Boxrud, D., Sambol, A., Abid, S., St. George, K., Bannerman, T., Moore, A., Stringer, D., Blevins, P., Demmler-Harrison, G., Ginsberg, M., Kriner, P., Waterman, S., Smole, S., Guevara, H., Belongia, E., Clark, P., Beatrice, S., Donis, R., Katz, J., Finelli, L., Bridges, C., Shaw, M., Jernigan, D., Uyeki, T., Smith, D., Klimov, A., & Cox, N. (2009). Antigenic and Genetic Characteristics of Swine-Origin 2009 A(H1N1) Influenza Viruses Circulating in Humans Science, 325 (5937), 197-201 DOI: 10.1126/science.1176225