Immunity to 2009 pandemic H1N1 among older populations

A recent study from Spain found that people born before 1950 show some immunity against 2009 pandemic influenza A H1N1. This is because H1N1 subtype viruses, similar to the 2009 pandemic H1N1 strain, have been circulating since the Spanish influenza pandemic in 1918.

So far, H1N1 subtype viruses have not dramatically changed in their genetic makeup over time, so individuals who were previously exposed to these strains will have more immunity to the 2009 H1N1 virus than younger people who have never been exposed to an H1N1 virus.

However, as might be expected, when older persons who are susceptible to H1N1 viruses (this is a minority of older people) become infected with 2009 pandemic H1N1, they seem to have a higher likelihood of needing hospital care and a higher case fatality rate than any other age group.

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