From March 24 to April 29, 2009, a total of 2155 cases of severe pneumonia, involving 821 hospitalizations and 100 H1N1 deaths, were reported to the Mexican Ministry of Health.
During the early phase of this influenza pandemic, there was a sudden increase in the rate of severe pneumonia and a shift in the age distribution of patients with such illness, which was reminiscent of past pandemics and suggested relative protection for persons who were exposed to H1N1 strains during childhood before the 1957 pandemic. If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations.
Severe Respiratory Disease Concurrent with the Circulation of H1N1 Influenza
http://www.nejm.org/doi/full/10.1056/NEJMoa0904023
Swine Flu Not an Accident From a Lab, W.H.O. Says
By DONALD G. McNEIL Jr.MAY 14, 2009
https://www.nytimes.com/2009/05/15/health/policy/15flu.html
"In response to a question, Fukuda said Gibbs suggested that the novel virus seems to have an increase in lysine (an amino acid) residues and that it sits out on a "long branch" of the influenza phylogenetic tree in comparison with other swine flu viruses. He said it is known that growing flu viruses in eggs can lead to an increase in lysine residues.
However, human, avian, and swine flu viruses differ somewhat in this respect, Fukuda said, adding, "After discussion and looking at older swine influenza viruses, it was concluded that the amount of lysine being seen was in fact very consistent with the natural increase in lysine being seen in swine flu viruses."
Also, the experts who evaluated the hypothesis said that because of historical gaps in swine flu genetic data, many swine flu viruses are somewhat isolated on the phylogenetic tree, so Gibbs' observation on this point was nothing unusual, according to Fukuda."
WHO rejects idea that novel H1N1 virus is lab-derived
http://www.cidrap.umn.edu/news-perspective/2009/05/who-rejects-idea-novel-h1n1-virus-lab-derived
" April 17, 2009, officials at the Centers for Disease Control and Prevention (CDC) confirmed two cases of swine influenza in children living in neighboring counties in California.1 Here we take a perspective from systems biology to review the series of evolutionary and epidemiologic events, starting in 1918, that led to the emergence of the current swine-origin influenza A (H1N1) strain (S-OIV), which is widely known as swine flu."
Historical Perspective — Emergence of Influenza A (H1N1) Viruses
https://www.ncbi.nlm.nih.gov/pubmed/19564632
