The H1N1 swine flu or 2009 flu pandemic was an influenza pandemic, and the second of the two pandemics involving H1N1 influenza virus (the first of them was the 1918 flu pandemic), albeit in a new version. First described in April 2009, the virus appeared to be a new strain of H1N1 which resulted when a previous triple reassortment of bird, swine and human flu viruses further combined with a Eurasian pig flu virus, leading to the term "H1N1 swine flu" to be used for this pandemic. The 2009 flu pandemic (H1N1 swine flu) was one of the famous influenza pandemics in history. Unlike most strains of influenza, H1N1 does not disproportionately infect adults older than 60 years; this was an unusual and characteristic feature of the H1N1 pandemic. The 2009 pandemic influenza H1N1 (Novel Influenza A H1N1) Hemagglutinin (HA) proteins and antibodies were the main research tools for this influenza pandemic.
The phylogenetic origin of the flu virus that caused the 2009 influenza pandemic 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 H1N1 swine flu.
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 H1N1 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 H1N1 swine flu, which is the highest alert level. This alert level means that the H1N1 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.
According to the Centers for Disease Control and Prevention (CDC), in humans the symptoms of the 2009 "H1N1 swine flu" H1N1 virus are similar to those of influenza and of influenza-like illness in general. Symptoms include fever, cough, sore throat, body aches, headache, chills and fatigue. The 2009 outbreak has shown an increased percentage of patients reporting diarrhea and vomiting. The 2009 H1N1 virus is not zoonotic H1N1 swine flu, as it is not transmitted from pigs to humans, but from person to person. Because these symptoms are not specific to H1N1 swine flu, a differential diagnosis of probable H1N1 swine flu requires not only symptoms, but also a high likelihood of H1N1 swine flu due to the person's recent history. For example, during the 2009 H1N1 swine flu outbreak in the United States, the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed H1N1 swine flu, or who were in one of the five U.S. states that have reported H1N1 swine flu cases or in Mexico during the seven days preceding their illness onset." A diagnosis of confirmed H1N1 swine flu requires laboratory testing of a respiratory sample (a simple nose and throat swab).
The most common cause of H1N1 swine flu death is respiratory failure. Other causes of death are pneumonia (leading to sepsis), high fever (leading to neurological problems), dehydration (from excessive vomiting and diarrhea), electrolyte imbalance and kidney failure. Fatalities are more likely in young children and the elderly.
The total data of 2009 flu (H1N1 swine flu) pandemic deaths is 14,286 from ECDC, and 18,036 from WHO.