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How Virulent Is the 2009 Influenza A H1N1 (Swine Flu) Virus?

We might have better T cell responses against H1N1 than we thought.

New pandemic strains of influenza virus typically are more virulent than those that have been circulating longer, presumably because they present novel antigens to hosts. In this new study, researchers showed that 2009 influenza A H1N1 virus does not contain many surface antigens that have been presented to B cells by other viral strains. This conclusion is consistent with previous findings that few people who were born after 1957 have neutralizing antibodies to 2009 H1N1 and that H1N1 infection has been more problematic among younger people. Neutralizing antibodies typically recognize viral surface antigens.

However, the current pandemic H1N1 virus thus far has not proved to be as virulent as was feared initially. The researchers found that, compared with B cells, T cells (particularly CD8⁺ cells) are much more likely to recognize 2009 H1N1 virus — especially nonsurface antigens. Whereas the surface antigens of 2009 H1N1 are quite novel, the internal antigens are less novel. Although T cell responses to influenza virus might not affect transmissibility of the virus, they likely will affect virulence.

Comment: 2009 H1N1 virus is more virulent than seasonal flu viruses in laboratory animals with no previous influenza immunity. This report suggests, however, that internal antigens of 2009 H1N1 are less novel than its surface antigens and elicit a vigorous T cell response that diminishes severity of infection. Alternatively, the H1N1 pandemic, which began in March 2009, might have been relatively benign until now simply because temperatures have been relatively warm. Flu viruses love cold dry air, and winter weather will be here soon.

— Anthony L. Komaroff, MD

Published in Journal Watch General Medicine December 15, 2009

Citation(s):

Greenbaum JA et al. Pre-existing immunity against swine-origin H1N1 influenza viruses in the general human population. Proc Natl Acad Sci U S A 2009 Dec 1; 106:20365. (http://dx.doi.org/10.1073/pnas.0911580106)

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