Swine Influenza - H1N1 Virus Pandemic History

H1N1 Virus Pandemic History

A study conducted in 2011, and published in the journal Nature, has managed to establish the evolutionary origin of the flu strain of swine origin (S-OIV).

The phylogenetic origin of the flu virus that caused the 2009 pandemics can be traced before 1918. Around 1918, the ancestral virus, of avian origin, crossed the species boundaries and infected humans as human H1N1. The same phenomenon took place soon after in America, where the human virus was infecting pigs; it led to the emergence of the H1N1 swine strain, which later became the classic swine flu.

The new human H1N1 flu strain of avian origin kept transmitting among human populations until around 1957, when there was a co-infection between this strain and the avian H1N1 in humans. A reassortment event led to the development of a new strain (H2N2). From this point onwards, no outbreaks of H1N1 were reported in humans until around 1976.

New events of reassortment were not reported until 1968, when the avian strain H1N1 infected humans again; this time the virus met the strain H2N2, and the reassortment originated the strain H3N2. This strain has remained as a stable flu strain until now.

The mid-1970s were important for the evolution of flu strains. First, the re-emergence of the human H1N1 strain became a seasonal strain. Then, a small outbreak of swine H1N1 occurred in humans, and finally, the human H2N2 strain apparently became extinct. Around 1979, the avian H1N1 strain infected pigs and gave rise to Euroasiatic swine flu and H1N1 Euroasiatic swine virus, which is still being transmitted in swine populations.

The critical moment for the 2009 outbreak was between 1990 and 1993. A triple reassortment event in a pig host of North American H1N1 swine virus, the human H3N2 virus and avian H1N1 virus generated the swine H1N2 strain. Finally, the last step in S-OIV history was in 2009, when the virus H1N2 co-infected a human host at the same time as the Euroasiatic H1N1 swine strain. This led to the emergence of a new human H1N1 strain, which caused the 2009 pandemic.

On June 11, 2009, the World Health Organization raised the worldwide pandemic alert level to Phase 6 for swine flu, which is the highest alert level. This alert level means that the swine flu had spread worldwide and there were cases of people with the virus in most countries. The pandemic level identifies the spread of the disease or virus and not necessarily the severity of the disease.

Swine flu spread very rapidly worldwide due to its high human-to-human transmission rate and due to the frequency of air travel.