Influenza A (H1N1) virus VS. SARS

Severe Acute Respiratory Syndrome (SARS; pronounced /sɑrz/, sarz) is a respiratory disease in humans which is caused by the SARS coronavirus (SARS-CoV).^[1] There has been one near pandemic to date, between the months of November 2002 and July 2003, with 8,096 known infected cases and 774 deaths (a case-fatality rate of 9.6%) worldwide being listed in the World Health Organization's (WHO) 21 April 2004 concluding report.^[2] Within a matter of weeks in early 2003, SARS spread from the Guangdong province of China to rapidly infect individuals in some 37 countries around the world.^[3]

Mortality by age group as of 8 May 2003 is below 1 percent for people aged 24 or younger, 6 percent for those 25 to 44, 15 percent in those 45 to 64 and more than 50 percent for those over 65.^[4] For comparison, the case fatality rate for influenza is usually around 0.6 percent (primarily among the elderly) but can rise as high as 33 percent in locally severe epidemics of new strains. The mortality rate of the primary viral pneumonia form is about 70 percent.

As of May 2006, the spread of SARS has been fully contained thanks to the efforts of the WHO, with the last infected human case seen in June 2003 (disregarding a laboratory induced infection case in 2004). However, SARS is not claimed to have been eradicated (unlike smallpox), as it may still be present in its natural host reservoirs (animal populations) and may potentially return into the human population in the future.

Influenza A virus subtype H1N1, also known as A(H1N1), is a subtype of influenzavirus A and the most common cause of influenza (flu) in humans. Some strains of H1N1 are endemic in humans, including the strain(s) responsible for the 1918 flu pandemic which killed 50–100 million people worldwide. Less virulent H1N1 strains still exist in the wild today, worldwide, causing a small fraction of all influenza-like illness and a large fraction of all seasonal influenza. H1N1 strains caused roughly half of all flu infections in 2006.^[1] Other strains of H1N1 are endemic in pigs and in birds.

Influenza A virus strains are categorized according to two proteins found on the surface of the virus: hemagglutinin (H) and neuraminidase (N). All influenza A viruses contain hemagglutinin and neuraminidase, but the structure of these proteins differs from strain to strain due to rapid genetic mutation in the viral genome.

Influenza A virus strains are assigned an H number and an N number based on which forms of these two proteins the strain contains. There are 16 H and 9 N subtypes known in birds, but only H 1, 2 and 3, and N 1 and 2 are commonly found in humans.