Abstract
Since May 2009, the pandemic influenza A(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 analysed 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, while in age groups 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 A(H1N1) virus was closely related to that of the Spanish influenza and 1976 swine influenza viruses. Based on the three-dimensional structure of the HA molecule, the antigenic epitopes of the pandemic virus HA are more closely related to those of the Spanish influenza HA than to those of recent seasonal influenza A(H1N1) 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.
Highlights
In March and April 2009, a previously unknown variant of influenza A(H1N1) virus was able to cause sporadic infection clusters and epidemics in North America [1] and Mexico [2]
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 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
Summary
In March and April 2009, a previously unknown variant of influenza A(H1N1) virus was able to cause sporadic infection clusters and epidemics in North America [1] and Mexico [2]. Compared with seasonal H1N1 viruses the novel virus is genetically and antigenically very different from human H1N1 viruses that have been circulating during the last 60 to 70 years [5]. Epidemiological analyses initially from Mexico [6] and the United States (US) [4], and later from Europe [7] and the southern hemisphere [8,9] revealed that the disease is affecting children, young adults and the general population under 65 years of age. At present only limited data on this is available from Europe
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