Since May 2009, the H1N1 virus has been spreading throughout the world. Epidemiological data indicate that the elderly are underrepresented among the ill individuals. Approximately 1,000 serum specimens collected in Finland in 2004 and 2005 from individuals born between 1909 and 2005, were analyzed by haemagglutination-inhibition test for the presence of antibodies against the 2009 pandemic influenza A(H1N1) and recently circulating seasonal influenza A viruses.

  • Ninety-six per cent of individuals born between 1909 and 1919 had antibodies against the 2009 pandemic influenza virus.
  • Those born between 1920 and 1944, the prevalence varied from 77% to 14%.
  • Most individuals born after 1944 lacked antibodies to the pandemic virus.

In sequence comparisons the haemagglutinin (HA) gene of the 2009 pandemic influenza H1N1 virus was closely related to that of the Spanish influenza and 1976 swine influenza viruses. Among the elderly, cross-reactive antibodies against the 2009 pandemic influenza virus, which likely originate from infections caused by the Spanish influenza virus and its immediate descendants, may provide protective immunity against the present pandemic virus.

This study demonstrates that in Finland, individuals born between 1909 and 1924 and to a lesser extent those born between 1925 and 1944 have pre-existing humoral immunity against the 2009 pandemic H1N1 influenza A virus.

Genetic and structural analyses also revealed that the 2009 pandemic virus is more closely related to the 1918 Spanish influenza and to the 1976 Fort Dix outbreak swine viruses than to any other seasonal H1N1-type influenza viruses that have been isolated since the 1930s. It is highly likely that immunity induced by the Spanish influenza virus, as seen in the oldest individuals included in this study, provides cross-protection against the currently circulating 2009 pandemic influenza virus.

In case the cross-reactivity against the 2009 pandemic influenza virus is indeed due to infections caused by the Spanish influenza and/or its immediate descendant viruses in the late 1910s and the 1920s, this would seem to suggest that specific anti-influenza immunity can last for an extremely long time, even a lifetime.

  • 33-55% of individuals who were born between the years 1909 and 1924 had relatively high antibody levels (≥40 HI titres) against the 2009 pandemic influenza virus and are thus likely to be protected against infection with this virus.
  • Antibody levels ≥40 as measured by the HI method are generally considered as protective and such post-vaccination antibody levels are an indication of an efficient vaccine-induced humoral immune response. There was also a very good correlation between the level of cross-reactivity in the older age groups and the evolution of the Spanish influenza virus descendants.
  • Even if there is a considerable gap in available virus isolates and HA sequences between the years 1918 and 1933 we can estimate the evolutionary speed of the virus to be at least 1% of HA1 amino acids changes per year.
  • Apparently, the evolution was so fast that the viruses circulating in the 1930s and 1940s were already quite distinct from the initial Spanish influenza virus (Figure 2, below) and thus infections caused by those viruses were unable to induce significant cross-reactivity against the 2009 pandemic influenza virus.

Apparently, The Evolution Was So Fast That The Viruses Circulating In The 1930S And 1940S Were Already Quite Distinct From The Initial Spanish Influenza Virus (Figure 2) And Thus Infections Caused By Those Viruses Were Unable To Induce Significant Cross-Reactivity Against The 2009 Pandemic Influenza Virus.

Eurosurveillance, Volume 15, Issue 5, 04 February 2010