The influenza pandemic the world was waiting for may have arrived on April 2009, although to date its virulence appears to be no greater than that of seasonal influenza. Mexico was the first country where there was a sharp increase in reports of patients requiring hospitalization for pneumonia and an unusual series of deaths, leading to the suspicion that a new influenza virus strain was in circulation. During the same period, officials at the Centres for Disease Control and Prevention (CDC), Atlanta, uncovered two cases of influenza, the so-called ‘swine influenza,’ that were clearly due to a novel strain; the two patients were children living in neighbouring counties in California.[3
The causal agent, in fact, is a swine-origin influenza A (H1N1) virus (S-OIV) that is characterized by a unique combination of gene segments that has not been previously identified among human or swine influenza A viruses. Actually, the new H1N1 virus appears to be a mixture of avian, porcine, and human influenza RNA. Genomic analysis indicates that it is closely related to common reassortant swine influenza A viruses that have been isolated in North America, Europe, and Asia in the last 20 years.[4
Historically, there have been four to five pandemics of influenza during the 20th
century, which have occurred at intervals of 9–39 years. The 1918 pandemic, caused by worldwide spread of a human influenza A (H1N1) virus, was responsible for 40–50 million deaths. An estimated 4.9 million excess deaths, representing 2% of the population, occurred in India alone.[7
] After the pandemic subsided, sporadic cases of human influenza H1N1 continued to occur worldwide. H1N1 then mysteriously disappeared in 1957, likely due to both competition with the emerging pandemic H2N2 strain as well as the development of immunity to H1N1 among populations.
On January 1976, an outbreak of a respiratory disease occurred among soldiers in an army base in Fort Dix, New Jersey. Two hundred and thirty individuals had serological evidence of infection and there was one death. A novel virus H1N1 A/New Jersey/76 was identified as the cause of the epidemic that, fortunately, did not extend outside the base.[8
In November 1977, another H1N1 strain re-emerged in the former Soviet Union, Hong Kong, and North-Eastern China. It caused a relatively mild disease, mostly in young people.[9
] Genetic studies showed that the virus causing the 1977 epidemic was similar to the one isolated in the year 1950, but significantly different from the influenza A (H1N1) strains isolated in 1947 and 1957. Thus, it is likely that the 1977 strain was one that had been preserved since 1950.[10
] The cause of the re-emergence was hypothesized to be due to accidental release from a laboratory, in the face of a waning population immunity to H1N1.[11
The current S-OIV lineage carries three gene segments that share, with the human seasonal virus, a common descent from the 1918 H1N1 virus. Indeed, whereas 1918 influenza A (H1N1) virus likely emerged simultaneously from birds to humans and swine, S-OIV probably emerged from swine to humans. This was likely the result of a reassortment between two influenza A (H1N1) swine viruses. These two viruses were actually the products of at least four independent avian-to-mammalian cross-species transmission. During this process of evolution there were at least four reassortments of gene segments among avian, human and swine-adapted viruses.[12
It is not clear, however, whether this sharing of genes segments between human seasonal influenza A (H3N2) and current seasonal influenza A (H1N1) will allow S-OIV-infected individuals to develop effective immunity and clinical protection against S-OIV itself.
The aim of this review is to give an update of the global situation of the 2009 S-OIV pandemic. To obtain the material presented in this article, we searched the PubMed database using the following key words: H1N1, S-OIV, and swine flu. We also visited the websites of the main international health agencies, namely the World Health Organization (WHO) and Centres for Disease Control (CDC), Atlanta, to search for relevant matter. The cut-off date for the search was August 10, 2009.