Influenza Pandemic - Other Pandemic Threat Subtypes

Other Pandemic Threat Subtypes

"Human influenza virus" usually refers to those subtypes that spread widely among humans. H1N1, H1N2, and H3N2 are the only known Influenza A virus subtypes currently circulating among humans.

Genetic factors in distinguishing between "human flu viruses" and "avian influenza viruses" include:

PB2: (RNA polymerase): Amino acid (or residue) position 627 in the PB2 protein encoded by the PB2 RNA gene. Until H5N1, all known avian influenza viruses had a Glu at position 627, while all human influenza viruses had a lysine.

HA: (hemagglutinin): Avian influenza HA bind alpha 2–3 sialic acid receptors while human influenza HA bind alpha 2–6 sialic acid receptors.

"About 52 key genetic changes distinguish avian influenza strains from those that spread easily among people, according to researchers in Taiwan, who analyzed the genes of more than 400 A type flu viruses." "How many mutations would make an avian virus capable of infecting humans efficiently, or how many mutations would render an influenza virus a pandemic strain, is difficult to predict. We have examined sequences from the 1918 strain, which is the only pandemic influenza virus that could be entirely derived from avian strains. Of the 52 species-associated positions, 16 have residues typical for human strains; the others remained as avian signatures. The result supports the hypothesis that the 1918 pandemic virus is more closely related to the avian influenza A virus than are other human influenza viruses."

Highly pathogenic H5N1 avian influenza kills 50% of humans that catch it. In one case, a boy with H5N1 experienced diarrhea followed rapidly by a coma without developing respiratory or flu-like symptoms.

The Influenza A virus subtypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

• H1N1 caused "Spanish Flu" and the 2009 swine flu outbreak (novel H1N1)
• H2N2 caused "Asian Flu"
• H3N2 caused "Hong Kong Flu"
• H5N1 is "bird flu", endemic in avians
• H7N7 has unusual zoonotic potential
• H1N2 is currently endemic in humans and pigs
• H9N2, H7N2, H7N3, H10N7

H1N1

H1N1 is currently endemic in both human and pig populations. A variant of H1N1 was responsible for the Spanish flu pandemic that killed some 50 million to 100 million people worldwide over about a year in 1918 and 1919. Controversy arose in October 2005, after the H1N1 genome was published in the journal, Science. Many fear that this information could be used for bioterrorism.

"When he compared the 1918 virus with today's human flu viruses, Dr. Taubenberger noticed that it had alterations in just 25 to 30 of the virus's 4,400 amino acids. Those few changes turned a bird virus into a killer that could spread from person to person."

In mid-April 2009, an H1N1 variant appeared in Mexico, with its center in Mexico City. By 26 April the variant had spread widely; with cases reported in Canada, the US, New Zealand, the UK, France, Spain and Israel. On 29 April WHO raised the worldwide pandemic phase to 5. On 11 June 2009 the WHO raised the worldwide pandemic phase to 6, which means that the H1N1 swine flu has reached pandemic proportions, with nearly 30,000 confirmed cases worldwide. 8 November 2009 worldwide update by the UN's World Health Organization (WHO) states that "206 countries and overseas territories/communities have officially reported over 503,536 laboratory confirmed cases of the influenza pandemic H1N1 infection, including 6,250 deaths."

1. Microscopic image of the H1N1 virus

2. Microscopic image of the H1N1 virus

Further information: 2009 swine flu outbreak

H2N2

The Asian Flu was a pandemic outbreak of H2N2 avian influenza that originated in China in 1957, spread worldwide that same year during which a influenza vaccine was developed, lasted until 1958 and caused between one and four million deaths.

H3N2

H3N2 is currently endemic in both human and pig populations. It evolved from H2N2 by antigenic shift and caused the Hong Kong Flu pandemic of 1968 and 1969 that killed up to 750,000."An early-onset, severe form of influenza A H3N2 made headlines when it claimed the lives of several children in the United States in late 2003."

The dominant strain of annual flu in January 2006 is H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005.

"ontemporary human H3N2 influenza viruses are now endemic in pigs in southern China and can reassort with avian H5N1 viruses in this intermediate host."

H7N7

H7N7 has unusual zoonotic potential. In 2003 in Netherlands 89 people were confirmed to have H7N7 influenza virus infection following an outbreak in poultry on several farms. One death was recorded.

H1N2

H1N2 is currently endemic in both human and pig populations. The new H1N2 strain appears to have resulted from the reassortment of the genes of the currently circulating influenza H1N1 and H3N2 subtypes. The hemagglutinin protein of the H1N2 virus is similar to that of the currently circulating H1N1 viruses and the neuraminidase protein is similar to that of the current H3N2 viruses.